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.
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