Add irregular verbs reference guides to grammar section

Add two grammar notes under new "Irregular Verbs" category:
- Most Common Irregular Verbs: 15 essential verbs with present
  and preterite forms plus example sentences
- Types of Irregular Verbs: spelling changes, stem changes, and
  unique irregulars with patterns and examples

Closes #5

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Conjuga/Conjuga/Models/GrammarNote.swift

@@ -29,6 +29,8 @@ struct GrammarNote: Identifiable {
         seConstructions,
         estarGerundProgressive,
         spanishSuffixes,
+        commonIrregularVerbs,
+        typesOfIrregularVerbs,
     ]
 
     // MARK: - 1. Ser vs Estar
@@ -1099,4 +1101,177 @@ struct GrammarNote: Identifiable {
         **Key insight:** Once you recognize root + suffix patterns, you can decode unfamiliar words. For example, *panadería* = *pan* (bread) + *-ero* (person) + *-ía* (place) = "place where the bread-person works" = bakery. The suffix system makes Spanish vocabulary highly predictable and composable.
         """
     )
+
+    // MARK: - 22. Most Common Irregular Verbs
+
+    private static let commonIrregularVerbs = GrammarNote(
+        id: "common-irregular-verbs",
+        title: "Most Common Irregular Verbs",
+        category: "Irregular Verbs",
+        body: """
+        These 15 verbs are among the most used in Spanish — and they are all irregular. Mastering their forms is essential because you will encounter them in nearly every conversation.
+
+        **ser** — to be (permanent)
+
+        *yo soy, tú eres, él es* — Present
+        *yo fui, tú fuiste, él fue* — Preterite
+        *Soy estudiante.* — I am a student.
+
+        **estar** — to be (temporary/location)
+
+        *yo estoy, tú estás, él está* — Present
+        *yo estuve, tú estuviste, él estuvo* — Preterite
+        *Estoy cansado.* — I am tired.
+
+        **ir** — to go
+
+        *yo voy, tú vas, él va* — Present
+        *yo fui, tú fuiste, él fue* — Preterite (same as ser!)
+        *Voy al supermercado.* — I'm going to the supermarket.
+
+        **haber** — to have (auxiliary)
+
+        *yo he, tú has, él ha* — Present
+        *yo hube, tú hubiste, él hubo* — Preterite
+        *He comido ya.* — I have already eaten.
+
+        **tener** — to have (possession)
+
+        *yo tengo, tú tienes, él tiene* — Present
+        *yo tuve, tú tuviste, él tuvo* — Preterite
+        *Tengo dos hermanos.* — I have two siblings.
+
+        **hacer** — to do / to make
+
+        *yo hago, tú haces, él hace* — Present
+        *yo hice, tú hiciste, él hizo* — Preterite
+        *¿Qué haces?* — What are you doing?
+
+        **poder** — to be able to / can
+
+        *yo puedo, tú puedes, él puede* — Present
+        *yo pude, tú pudiste, él pudo* — Preterite
+        *No puedo dormir.* — I can't sleep.
+
+        **querer** — to want / to love
+
+        *yo quiero, tú quieres, él quiere* — Present
+        *yo quise, tú quisiste, él quiso* — Preterite
+        *Quiero agua, por favor.* — I want water, please.
+
+        **decir** — to say / to tell
+
+        *yo digo, tú dices, él dice* — Present
+        *yo dije, tú dijiste, él dijo* — Preterite
+        *¿Qué dices?* — What are you saying?
+
+        **venir** — to come
+
+        *yo vengo, tú vienes, él viene* — Present
+        *yo vine, tú viniste, él vino* — Preterite
+        *Vengo de la tienda.* — I'm coming from the store.
+
+        **saber** — to know (facts)
+
+        *yo sé, tú sabes, él sabe* — Present
+        *yo supe, tú supiste, él supo* — Preterite
+        *No sé la respuesta.* — I don't know the answer.
+
+        **poner** — to put / to place
+
+        *yo pongo, tú pones, él pone* — Present
+        *yo puse, tú pusiste, él puso* — Preterite
+        *Pon el libro en la mesa.* — Put the book on the table.
+
+        **salir** — to leave / to go out
+
+        *yo salgo, tú sales, él sale* — Present
+        *yo salí, tú saliste, él salió* — Preterite
+        *Salgo a las ocho.* — I leave at eight.
+
+        **dar** — to give
+
+        *yo doy, tú das, él da* — Present
+        *yo di, tú diste, él dio* — Preterite
+        *Dame un momento.* — Give me a moment.
+
+        **ver** — to see
+
+        *yo veo, tú ves, él ve* — Present
+        *yo vi, tú viste, él vio* — Preterite
+        *¿Ves eso?* — Do you see that?
+
+        **Tip:** These verbs appear so frequently that you will naturally memorize them through practice. Focus on the present and preterite forms first — those are the ones you need every day.
+        """
+    )
+
+    // MARK: - 23. Types of Irregular Verbs
+
+    private static let typesOfIrregularVerbs = GrammarNote(
+        id: "types-of-irregular-verbs",
+        title: "Types of Irregular Verbs",
+        category: "Irregular Verbs",
+        body: """
+        Spanish irregular verbs follow predictable patterns. Once you learn the pattern, you can conjugate dozens of verbs correctly. There are three main types of irregularity.
+
+        **Spelling Changes**
+
+        These verbs change their spelling to preserve the correct pronunciation. The sound stays the same — only the letters change. This happens because Spanish spelling rules require certain letter combinations before specific vowels.
+
+        *c → qu* (before e): buscar → busqué, tocar → toqué, sacar → saqué
+        *g → gu* (before e): pagar → pagué, llegar → llegué, jugar → jugué
+        *z → c* (before e): empezar → empecé, almorzar → almorcé, cruzar → crucé
+        *g → j* (before a/o): coger → cojo, elegir → elijo, proteger → protejo
+        *gu → gü* (before e): averiguar → averigüé
+        *i → y* (between vowels): leer → leyó/leyeron, oír → oyó/oyeron
+
+        *Busqué mis llaves por toda la casa.* — I looked for my keys all over the house.
+        *Llegué tarde al trabajo.* — I arrived late to work.
+        *Empecé a estudiar español el año pasado.* — I started studying Spanish last year.
+
+        **Stem Changes**
+
+        These verbs change a vowel in the stem when it is stressed. The change only happens in certain persons (usually all except nosotros and vosotros in the present tense). Common patterns:
+
+        *e → ie:* pensar → pienso, querer → quiero, preferir → prefiero, cerrar → cierro, entender → entiendo
+
+        *Quiero ir al cine.* — I want to go to the movies.
+        *Prefiero quedarme en casa.* — I prefer to stay at home.
+
+        *o → ue:* poder → puedo, dormir → duermo, volver → vuelvo, encontrar → encuentro, recordar → recuerdo
+
+        *No puedo encontrar mi teléfono.* — I can't find my phone.
+        *Duermo ocho horas cada noche.* — I sleep eight hours every night.
+
+        *e → i* (only -ir verbs): pedir → pido, servir → sirvo, repetir → repito, seguir → sigo, vestirse → me visto
+
+        *Pido una cerveza, por favor.* — I'll have a beer, please.
+        *Sigo estudiando todos los días.* — I keep studying every day.
+
+        *u → ue:* jugar → juego (the only verb with this pattern!)
+
+        *Juego al fútbol los sábados.* — I play soccer on Saturdays.
+
+        **Unique Irregulars**
+
+        These verbs have forms that don't follow any predictable pattern — you simply have to memorize them. The good news is that the most common ones (ser, ir, haber, etc.) are used so frequently that you learn them quickly through exposure.
+
+        *ser:* soy, eres, es, somos, sois, son (present) — fui, fuiste, fue... (preterite)
+        *ir:* voy, vas, va, vamos, vais, van (present) — fui, fuiste, fue... (preterite, same as ser!)
+        *haber:* he, has, ha, hemos, habéis, han (present auxiliary)
+        *hacer:* hago (yo present), hice/hizo (preterite)
+        *decir:* digo (yo present), dije/dijo (preterite)
+        *tener:* tengo (yo present), tuve (preterite)
+        *venir:* vengo (yo present), vine (preterite)
+
+        **The "go" verbs — yo forms ending in -go:** Many common verbs have an irregular *yo* form in the present tense that ends in *-go*, while all other persons are regular or follow a stem change.
+
+        *tener → tengo, poner → pongo, salir → salgo, venir → vengo, hacer → hago, decir → digo, traer → traigo, oír → oigo, caer → caigo*
+
+        *Tengo que salir ahora.* — I have to leave now.
+        *Pongo la mesa antes de cenar.* — I set the table before dinner.
+
+        **Tip:** Use the Irregularity Drills in the Practice tab to focus on each type separately — spelling changes, stem changes, or unique irregulars.
+        """
+    )
 }