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/androidMain/kotlin/com/example/casera/network/ApiClient.android.kt

@@ -8,6 +8,7 @@ import io.ktor.client.plugins.logging.*
 import io.ktor.serialization.kotlinx.json.*
 import kotlinx.serialization.json.Json
 import java.util.Locale
+import java.util.TimeZone
 
 actual fun getLocalhostAddress(): String = "10.0.2.2"
 
@@ -15,6 +16,10 @@ actual fun getDeviceLanguage(): String {
     return Locale.getDefault().language
 }
 
+actual fun getDeviceTimezone(): String {
+    return TimeZone.getDefault().id
+}
+
 actual fun createHttpClient(): HttpClient {
     return HttpClient(OkHttp) {
         install(ContentNegotiation) {
@@ -32,6 +37,7 @@ actual fun createHttpClient(): HttpClient {
 
         install(DefaultRequest) {
             headers.append("Accept-Language", getDeviceLanguage())
+            headers.append("X-Timezone", getDeviceTimezone())
         }
     }
 }