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>

ui/src/components/SettingsModal.tsx

@@ -325,7 +325,8 @@ export function SettingsModal({ isOpen, onClose }: SettingsModalProps) {
                           : 'bg-background text-foreground hover:bg-muted'
                       } ${isSaving ? 'opacity-50 cursor-not-allowed' : ''}`}
                     >
-                      {model.name}
+                      <span className="block">{model.name}</span>
+                      <span className="block text-xs opacity-60">{model.id}</span>
                     </button>
                   ))}
                 </div>