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/jsMain/kotlin/com/example/casera/network/ApiClient.js.kt

@@ -15,6 +15,11 @@ actual fun getDeviceLanguage(): String {
     return window.navigator.language.split("-").firstOrNull() ?: "en"
 }
 
+actual fun getDeviceTimezone(): String {
+    // Use Intl API to get IANA timezone name (e.g., "America/Los_Angeles")
+    return js("Intl.DateTimeFormat().resolvedOptions().timeZone") as String
+}
+
 actual fun createHttpClient(): HttpClient {
     return HttpClient(Js) {
         install(ContentNegotiation) {
@@ -32,6 +37,7 @@ actual fun createHttpClient(): HttpClient {
 
         install(DefaultRequest) {
             headers.append("Accept-Language", getDeviceLanguage())
+            headers.append("X-Timezone", getDeviceTimezone())
         }
     }
 }