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>

registry.py

@@ -46,10 +46,16 @@ def _migrate_registry_dir() -> None:
 # Available models with display names
 # To add a new model: add an entry here with {"id": "model-id", "name": "Display Name"}
 AVAILABLE_MODELS = [
-    {"id": "claude-opus-4-5-20251101", "name": "Claude Opus 4.5"},
-    {"id": "claude-sonnet-4-5-20250929", "name": "Claude Sonnet 4.5"},
+    {"id": "claude-opus-4-6", "name": "Claude Opus"},
+    {"id": "claude-sonnet-4-5-20250929", "name": "Claude Sonnet"},
 ]
 
+# Map legacy model IDs to their current replacements.
+# Used by get_all_settings() to auto-migrate stale values on first read after upgrade.
+LEGACY_MODEL_MAP = {
+    "claude-opus-4-5-20251101": "claude-opus-4-6",
+}
+
 # List of valid model IDs (derived from AVAILABLE_MODELS)
 VALID_MODELS = [m["id"] for m in AVAILABLE_MODELS]
 
@@ -59,7 +65,7 @@ VALID_MODELS = [m["id"] for m in AVAILABLE_MODELS]
 _env_default_model = os.getenv("ANTHROPIC_DEFAULT_OPUS_MODEL")
 if _env_default_model is not None:
     _env_default_model = _env_default_model.strip()
-DEFAULT_MODEL = _env_default_model or "claude-opus-4-5-20251101"
+DEFAULT_MODEL = _env_default_model or "claude-opus-4-6"
 
 # Ensure env-provided DEFAULT_MODEL is in VALID_MODELS for validation consistency
 # (idempotent: only adds if missing, doesn't alter AVAILABLE_MODELS semantics)
@@ -598,6 +604,9 @@ def get_all_settings() -> dict[str, str]:
     """
     Get all settings as a dictionary.
 
+    Automatically migrates legacy model IDs (e.g. claude-opus-4-5-20251101 -> claude-opus-4-6)
+    on first read after upgrade. This is a one-time silent migration.
+
     Returns:
         Dictionary mapping setting keys to values.
     """
@@ -606,7 +615,26 @@ def get_all_settings() -> dict[str, str]:
         session = SessionLocal()
         try:
             settings = session.query(Settings).all()
-            return {s.key: s.value for s in settings}
+            result = {s.key: s.value for s in settings}
+
+            # Auto-migrate legacy model IDs
+            migrated = False
+            for key in ("model", "api_model"):
+                old_id = result.get(key)
+                if old_id and old_id in LEGACY_MODEL_MAP:
+                    new_id = LEGACY_MODEL_MAP[old_id]
+                    setting = session.query(Settings).filter(Settings.key == key).first()
+                    if setting:
+                        setting.value = new_id
+                        setting.updated_at = datetime.now()
+                        result[key] = new_id
+                        migrated = True
+                        logger.info("Migrated setting '%s': %s -> %s", key, old_id, new_id)
+
+            if migrated:
+                session.commit()
+
+            return result
         finally:
             session.close()
     except Exception as e:
@@ -624,10 +652,10 @@ API_PROVIDERS: dict[str, dict[str, Any]] = {
         "base_url": None,
         "requires_auth": False,
         "models": [
-            {"id": "claude-opus-4-5-20251101", "name": "Claude Opus 4.5"},
-            {"id": "claude-sonnet-4-5-20250929", "name": "Claude Sonnet 4.5"},
+            {"id": "claude-opus-4-6", "name": "Claude Opus"},
+            {"id": "claude-sonnet-4-5-20250929", "name": "Claude Sonnet"},
         ],
-        "default_model": "claude-opus-4-5-20251101",
+        "default_model": "claude-opus-4-6",
     },
     "kimi": {
         "name": "Kimi K2.5 (Moonshot)",