Add Simplified Chinese (zh) localization (#901)

* feat: add Simplified Chinese (zh) localization

- Add complete values-zh/strings.xml (921 strings, all keys covered)
- Translate 107 hardcoded UI strings across 34 Kotlin files
- Add 简体中文 option to in-app language picker
- Terminology aligned with official Hermes dashboard zh translations

* fix: resource-ize hardcoded Chinese strings per review

Address Hy4ri's review on PR #901:
- Move 104 hardcoded Chinese UI strings to string resources
  (English in values/strings.xml, Chinese in values-zh/strings.xml)
  referenced via stringResource()
- SystemViewModel labels -> @StringRes + AndroidViewModel with
  resource-based toast templates
- Make TodoTaskCardTest assertion locale-agnostic via getString()
- Update cookie tests to assert new host-agnostic session cookie
  behavior (WiFi/Tailscale switch-safe)
- Session cookie mirroring to WILDCARD bucket survives host switches

* style: apply ktlintFormat per review

---------

Co-authored-by: hermes-i18n <hermes-i18n@localhost>

app/src/main/java/com/m57/hermescontrol/ui/webhooks/WebhooksScreen.kt

@@ -161,7 +161,7 @@ fun WebhooksScreen(
                         value = state.createFormDescription,
                         onValueChange = viewModel::updateCreateFormDescription,
                         label = { Text(stringResource(R.string.webhooks_field_description)) },
-                        placeholder = { Text("What this webhook does...") },
+                        placeholder = { Text(stringResource(R.string.webhooks_placeholder)) },
                         singleLine = true,
                         modifier = Modifier.fillMaxWidth(),
                     )
@@ -291,7 +291,7 @@ fun WebhooksScreen(
                                     )
                                     state.baseUrl?.let {
                                         Text(
-                                            text = "Receiver URL: $it",
+                                            text = stringResource(R.string.webhooks_receiver_url, it),
                                             style = MaterialTheme.typography.bodySmall,
                                             color = MaterialTheme.colorScheme.onSurfaceVariant,
                                             maxLines = 1,
@@ -446,7 +446,7 @@ private fun SubscriptionCard(
                         Spacer(modifier = Modifier.width(6.dp))
                         Icon(
                             imageVector = Icons.Filled.Lock,
-                            contentDescription = "Secret configured",
+                            contentDescription = stringResource(R.string.webhooks_secret_configured),
                             modifier = Modifier.size(14.dp),
                             tint = MaterialTheme.colorScheme.primary,
                         )
@@ -482,7 +482,7 @@ private fun SubscriptionCard(
                     ) {
                         Icon(
                             imageVector = Icons.Filled.Delete,
-                            contentDescription = "Delete subscription",
+                            contentDescription = stringResource(R.string.webhooks_delete_subscription),
                             modifier = Modifier.size(18.dp),
                             tint = MaterialTheme.colorScheme.error.copy(alpha = 0.7f),
                         )