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/test/java/com/m57/hermescontrol/E2eIntegrationTest.kt

@@ -1182,7 +1182,7 @@ class E2eIntegrationTest {
                 Response.success(UpdateCheckResponse())
             coEvery { mockApiService.triggerBackup(BackupTriggerRequest()) } returns Response.success(ActionResponse())
 
-            val viewModel = SystemViewModel()
+            val viewModel = SystemViewModel(mockApp)
             viewModel.loadAll()
             advanceUntilIdle()