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/providers/ProvidersScreen.kt

@@ -188,7 +188,7 @@ private fun ProviderCard(
                 provider.status.sourceLabel?.let { label ->
                     Spacer(modifier = Modifier.height(2.dp))
                     Text(
-                        text = "via $label",
+                        text = stringResource(R.string.providers_via, label),
                         style = MaterialTheme.typography.labelSmall,
                         color = MaterialTheme.colorScheme.onSurfaceVariant.copy(alpha = 0.6f),
                     )