Fixes #21 — Curated YouTube videos per guide + grammar item

Each of the 56 tense guides and grammar notes gets a curated YouTube video
attached (54 with picks, 2 silent nulls on rare / hard-to-find topics).
Users can stream in YouTube/Safari, download via YouTubeKit for offline
viewing, or play the local MP4 full-screen via AVPlayer.

YouTubeVideoStore loads the bundled youtube_videos.json at launch and serves
lookups by tense id or grammar note id. VideoDownloadService resolves the
best progressive MP4 stream off the main actor (YouTubeKit isn't Sendable),
writes to documents/videos/<videoId>.mp4, and records a DownloadedVideo row
in the local SwiftData container so the app knows what's on disk across
launches.

VideoActionsButtonRow is the unified UI for both detail views: three large
buttons — Stream (red, always enabled), Download (blue, disabled while in
flight and after completion, shows progress), Play (green, enabled only
when downloaded). Full-screen cover on tap. Settings gains a Downloaded
Videos list with swipe-delete, total-size summary, and a 500 MB warning.

Local store reset version bumped to 4 for the new DownloadedVideo schema.

Known fragility: YouTubeKit scrapes YouTube's private stream API and will
break when YouTube changes their internal format. Streaming keeps working
regardless.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Conjuga/SharedModels/Sources/SharedModels/DownloadedVideo.swift

@@ -0,0 +1,25 @@
+import Foundation
+import SwiftData
+
+/// Persistent record of a YouTube video downloaded to the device (Issue #21).
+/// Files live in the app's documents directory under `videos/<videoId>.mp4`;
+/// this model tracks the metadata needed to locate, display, and manage them.
+///
+/// Lives in the local store, not CloudKit — downloads are per-device.
+@Model
+public final class DownloadedVideo {
+    /// YouTube video ID — the primary key (unique).
+    @Attribute(.unique) public var videoId: String = ""
+    public var title: String = ""
+    public var filename: String = ""
+    public var byteCount: Int = 0
+    public var downloadedAt: Date = Date()
+
+    public init(videoId: String, title: String, filename: String, byteCount: Int) {
+        self.videoId = videoId
+        self.title = title
+        self.filename = filename
+        self.byteCount = byteCount
+        self.downloadedAt = Date()
+    }
+}