feat(backend): add icon fields to Provider model and default mappings

Extend Rust backend to support provider icon customization:

## Provider Model (src-tauri/src/provider.rs)
- Add `icon: Option<String>` field for icon name
- Add `icon_color: Option<String>` field for hex color
- Use serde rename `iconColor` for frontend compatibility
- Apply skip_serializing_if for clean JSON output
- Update Provider::new() to initialize icon fields as None

## Provider Defaults (src-tauri/src/provider_defaults.rs) [NEW]
- Define ProviderIcon struct with name and color fields
- Create DEFAULT_PROVIDER_ICONS static HashMap with 23 providers:
  - AI providers: OpenAI, Anthropic, Claude, Google, Gemini,
    DeepSeek, Kimi, Moonshot, Zhipu, MiniMax, Baidu, Alibaba,
    Tencent, Meta, Microsoft, Cohere, Perplexity, Mistral, HuggingFace
  - Cloud platforms: AWS, Azure, Huawei, Cloudflare
- Implement infer_provider_icon() with exact and fuzzy matching
- Add unit tests for matching logic (exact, fuzzy, case-insensitive)

## Deep Link Support (src-tauri/src/deeplink.rs)
- Initialize icon fields when creating Provider from deep link import

## Module Registration (src-tauri/src/lib.rs)
- Register provider_defaults module

## Dependencies (Cargo.toml)
- Add once_cell for lazy static initialization

This backend support enables icon persistence and future features
like auto-icon inference during provider creation.

src-tauri/Cargo.lock

@@ -616,6 +616,7 @@ dependencies = [
  "log",
  "objc2 0.5.2",
  "objc2-app-kit 0.2.2",
+ "once_cell",
  "regex",
  "reqwest",
  "rquickjs",