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/commonMain/kotlin/com/example/casera/network/ApiClient.kt

@@ -16,6 +16,13 @@ expect fun createHttpClient(): HttpClient
  */
 expect fun getDeviceLanguage(): String
 
+/**
+ * Get the device's timezone identifier (e.g., "America/Los_Angeles", "Europe/London").
+ * This is used to set the X-Timezone header for API requests
+ * so the server can calculate overdue tasks correctly based on the user's local time.
+ */
+expect fun getDeviceTimezone(): String
+
 object ApiClient {
     val httpClient = createHttpClient()