Add visual theme system for provider presets with custom icons and brand colors:
- Add PresetTheme interface supporting icon type and custom colors
- Claude Official: Claude brand icon + orange theme (#D97757)
- Codex Official: Codex brand icon + dark gray theme (#1F2937)
- Other presets: Default to theme blue (bg-blue-500)
- Custom config: Uses theme blue for consistency
Technical changes:
- Extend ProviderPreset and CodexProviderPreset interfaces with optional theme field
- Update ProviderPresetSelector to render icons and apply theme colors
- Support both Tailwind classes and hex colors via inline styles
- Remove unused Link import from ProviderCard
This restores the unique visual identity for official providers while
maintaining a unified theme color for third-party presets.
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