Fix inconsistent modal overlays by migrating all custom implementations
to the unified shadcn/ui Dialog component with proper z-index layering.
Changes:
- Update Dialog component to support three z-index levels:
- base (z-40): First-level dialogs
- nested (z-50): Nested dialogs
- alert (z-[60]): Alert/confirmation dialogs (using Tailwind arbitrary value)
- Refactor all custom modal implementations to use Dialog:
- EndpointSpeedTest: API endpoint speed testing panel
- ClaudeConfigEditor: Claude common config editor
- CodexQuickWizardModal: Codex quick setup wizard
- CodexCommonConfigModal: Codex common config editor
- SettingsDialog: Restart confirmation prompt
- Remove custom backdrop implementations and manual z-index
- Leverage Radix UI Portal for automatic DOM order management
- Ensure consistent overlay behavior and keyboard interactions
This eliminates the "background residue" issue where overlays from
different layers would conflict, providing a unified and professional
user experience across all modal interactions.
Introduce `closeAfterSave` callback to distinguish between save-and-close and cancel-and-close scenarios. Previously, saving settings would trigger `resetSettings()`, causing language changes to revert and requiring users to save twice for the change to take effect.
Changes:
- Add `closeAfterSave()`: close dialog without resetting settings after successful save
- Keep `closeDialog()`: reset settings when canceling or directly closing dialog
- Update save flow to use `closeAfterSave()` in `handleSaveClick`, `handleRestartLater`, and `handleRestartNow`
