refactor: make Settings UI the single source of truth for API provider

Remove legacy env-var-based provider/mode detection that caused misleading
UI badges (e.g., GLM badge showing when Settings was set to Claude).

Key changes:
- Remove _is_glm_mode() and _is_ollama_mode() env-var sniffing functions
  from server/routers/settings.py; derive glm_mode/ollama_mode purely from
  the api_provider setting
- Remove `import os` from settings router (no longer needed)
- Update schema comments to reflect settings-based derivation
- Remove "(configured via .env)" from badge tooltips in App.tsx
- Remove Kimi/GLM/Ollama/Playwright-headless sections from .env.example;
  add note pointing to Settings UI
- Update CLAUDE.md and README.md documentation to reference Settings UI
  for alternative provider configuration
- Update model IDs from claude-opus-4-5-20251101 to claude-opus-4-6
  across registry, client, chat sessions, tests, and UI defaults
- Add LEGACY_MODEL_MAP with auto-migration in get_all_settings()
- Show model ID subtitle in SettingsModal model selector
- Add Vertex passthrough test for claude-opus-4-6 (no date suffix)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/services/spec_chat_session.py

@@ -140,11 +140,11 @@ class SpecChatSession:
         system_cli = shutil.which("claude")
 
         # Build environment overrides for API configuration
-        from registry import get_effective_sdk_env
+        from registry import DEFAULT_MODEL, get_effective_sdk_env
         sdk_env = get_effective_sdk_env()
 
         # Determine model from SDK env (provider-aware) or fallback to env/default
-        model = sdk_env.get("ANTHROPIC_DEFAULT_OPUS_MODEL") or os.getenv("ANTHROPIC_DEFAULT_OPUS_MODEL", "claude-opus-4-5-20251101")
+        model = sdk_env.get("ANTHROPIC_DEFAULT_OPUS_MODEL") or os.getenv("ANTHROPIC_DEFAULT_OPUS_MODEL", DEFAULT_MODEL)
 
         try:
             self.client = ClaudeSDKClient(