Implement proper localization infrastructure (i18n) with .resw resource files

[shanselman]

Overview

Implement proper WinUI localization using .resw resource files so any language can be added by contributing a single resource file — no source code changes needed.

Prerequisite: #41 (settings persistence fix) — merged or in progress.

Phase 1: Extract English strings to Resources.resw

XAML strings

• Create src/OpenClaw.Tray.WinUI/Strings/en-us/Resources.resw
• Add x:Uid attributes to all XAML elements with user-visible text in:
  • SettingsWindow.xaml (headers, labels, button text, toggle headers, checkbox content)
  • StatusDetailWindow.xaml (section headers, default text, button text)
  • ActivityStreamWindow.xaml (headers, filter labels, button text)
  • NotificationHistoryWindow.xaml (headers, empty state text)
  • WebChatWindow.xaml (placeholder text)
• Pattern: <TextBlock x:Uid="StatusHeader" Text="STATUS"/> with StatusHeader.Text = STATUS in .resw

C# runtime strings

• Create a LocalizationHelper static class with ResourceLoader for C# strings
• Replace hardcoded strings in:
  • App.xaml.cs: menu items (Settings, Status, Chat, etc.), status labels, channel icons
  • WelcomeDialog.cs: title, body text, button labels
  • UpdateDialog.cs: title, version text, button labels
  • QuickSendDialog.cs: title, placeholder, button text, status messages
  • SettingsWindow.xaml.cs: runtime status strings (Testing..., Connected!, Failed)
  • StatusDetailWindow.xaml.cs: Last check: prefix, Unknown fallback
• Map ConnectionStatus enum to localized display strings instead of raw .ToString()

Ensure culture-invariant persistence

• Verify all settings save/load uses stable keys (see fix: NotificationSound persisted by display text instead of stable key #41)
• Verify no .ToString() on enums produces culture-dependent persisted values
• Use CultureInfo.InvariantCulture for any non-display formatting

Phase 2: Add Chinese Simplified translation

• Create src/OpenClaw.Tray.WinUI/Strings/zh-cn/Resources.resw
• Populate with translations from @Kobe9312's work in Add Chinese (Simplified) localization #30
• Windows automatically selects language based on OS locale
• Verify settings saved on Chinese OS load correctly on English OS

Phase 3: Validation

• Build and run tests
• Verify no mixed-language UI on Chinese locale
• Verify settings round-trip across locales
• Verify WinForms tray is unaffected (shares settings.json)

Out of scope

• WinForms tray localization (evaluate separately — may be sunset)
• Right-to-left layout (future issue)
• Localizing code comments or README (keep English; translated READMEs as separate files are fine)

References

• Closed PR Add Chinese (Simplified) localization #30 (Chinese translation — source-level approach)
• WinUI localization docs
• x:Uid directive

/cc @Kobe9312

[shanselman]

Implemented in PR #46 (merged). .resw resource files for en-US and zh-CN, x:Uid attributes, LocalizationHelper, all code-behind wired up. Any language can now be added with a single .resw file.