fix: address PR #163 review findings

- Fix model selection regression: _get_settings_defaults() now checks
  api_model (set by new provider UI) before falling back to legacy
  model setting, ensuring Claude model selection works end-to-end
- Add input validation for provider settings: api_base_url must start
  with http:// or https:// (max 500 chars), api_auth_token max 500
  chars, api_model max 200 chars
- Fix terminal.py misleading import alias: replace
  is_valid_project_name aliased as validate_project_name with direct
  is_valid_project_name import across all 5 call sites

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

server/routers/agent.py

@@ -32,7 +32,7 @@ def _get_settings_defaults() -> tuple[bool, str, int, bool, int]:
 
     settings = get_all_settings()
     yolo_mode = (settings.get("yolo_mode") or "false").lower() == "true"
-    model = settings.get("model", DEFAULT_MODEL)
+    model = settings.get("api_model") or settings.get("model", DEFAULT_MODEL)
 
     # Parse testing agent settings with defaults
     try: