Add X-Timezone header to all API requests

Send the device's IANA timezone identifier (e.g., "America/Los_Angeles")
with every API request to enable timezone-aware overdue task detection.

Platform implementations:
- Android/JVM: TimeZone.getDefault().id
- iOS: NSTimeZone.localTimeZone.name
- JS/WASM: Intl.DateTimeFormat().resolvedOptions().timeZone

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

composeApp/src/iosMain/kotlin/com/example/casera/network/ApiClient.ios.kt

@@ -8,6 +8,7 @@ import io.ktor.client.plugins.logging.*
 import io.ktor.serialization.kotlinx.json.*
 import kotlinx.serialization.json.Json
 import platform.Foundation.NSLocale
+import platform.Foundation.NSTimeZone
 import platform.Foundation.preferredLanguages
 
 actual fun getLocalhostAddress(): String = "127.0.0.1"
@@ -19,6 +20,10 @@ actual fun getDeviceLanguage(): String {
     return firstLanguage?.split("-")?.firstOrNull() ?: "en"
 }
 
+actual fun getDeviceTimezone(): String {
+    return NSTimeZone.localTimeZone.name
+}
+
 actual fun createHttpClient(): HttpClient {
     return HttpClient(Darwin) {
         install(ContentNegotiation) {
@@ -36,6 +41,7 @@ actual fun createHttpClient(): HttpClient {
 
         install(DefaultRequest) {
             headers.append("Accept-Language", getDeviceLanguage())
+            headers.append("X-Timezone", getDeviceTimezone())
         }
 
         engine {