refactor(models): migrate to granular model configuration architecture

Upgrade Claude model configuration from dual-key to quad-key system for
better model tier differentiation.

**Breaking Changes:**
- Replace `ANTHROPIC_SMALL_FAST_MODEL` with three granular keys:
  - `ANTHROPIC_DEFAULT_HAIKU_MODEL`
  - `ANTHROPIC_DEFAULT_SONNET_MODEL`
  - `ANTHROPIC_DEFAULT_OPUS_MODEL`

**Backend (Rust):**
- Add `normalize_claude_models_in_value()` for automatic migration
- Implement fallback chain: `DEFAULT_* || SMALL_FAST || MODEL`
- Auto-cleanup: remove legacy `SMALL_FAST` key after normalization
- Apply normalization across 6 critical paths:
  - Add/update provider
  - Read from live config
  - Write to live config
  - Refresh config snapshot

**Frontend (React):**
- Expand UI from 2 to 4 model input fields
- Implement smart fallback in `useModelState` hook
- Update `useKimiModelSelector` for Kimi model picker
- Add i18n keys for Haiku/Sonnet/Opus labels (zh/en)

**Configuration:**
- Update all 7 provider presets to new format
- DeepSeek/Qwen/Moonshot: use same model for all tiers
- Zhipu: preserve tier differentiation (glm-4.5-air for Haiku)

**Backward Compatibility:**
- Old configs auto-upgrade on first read/write
- Fallback chain ensures graceful degradation
- No manual migration required

Closes #[issue-number]

src/i18n/locales/en.json

@@ -258,6 +258,9 @@
     "visitWebsite": "Visit {{url}}",
     "anthropicModel": "Main Model",
     "anthropicSmallFastModel": "Fast Model",
+    "anthropicDefaultHaikuModel": "Default Haiku Model",
+    "anthropicDefaultSonnetModel": "Default Sonnet Model",
+    "anthropicDefaultOpusModel": "Default Opus Model",
     "modelPlaceholder": "GLM-4.6",
     "smallModelPlaceholder": "GLM-4.5-Air",
     "modelHelper": "Optional: Specify default Claude model to use, leave blank to use system default.",
@@ -373,6 +376,9 @@
     "modelConfig": "Model Configuration",
     "mainModel": "Main Model",
     "fastModel": "Fast Model",
+    "haikuModel": "Default Haiku",
+    "sonnetModel": "Default Sonnet",
+    "opusModel": "Default Opus",
     "refreshModels": "Refresh Model List",
     "pleaseSelectModel": "Please select a model",
     "noModels": "No models available",