feat: add unique icons and colors for preset providers

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.

src/components/providers/ProviderCard.tsx

@@ -1,5 +1,5 @@
 import { useMemo } from "react";
-import { GripVertical, Link } from "lucide-react";
+import { GripVertical } from "lucide-react";
 import { useTranslation } from "react-i18next";
 import type {
   DraggableAttributes,