Make Settings immutable (frozen=True) and fix all test mutations ...

- Add frozen=True to SettingsConfigDict in navi/config.py
- Convert model_validator to mode="before" since mode="after" cannot mutate frozen instances
- Replace all field-level monkeypatches in tests with whole-Settings object replacement
- Ensure cross-module settings consistency (content_store, session_files, share_file, content_publish, filesystem)

392 passed, 1 skipped

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Fix session file URLs (add /api prefix) and WebGL error handling in STL viewer ...

Backend:
- _file_url in content_store.py now returns /api/sessions/... instead of /sessions/...
- share_file.py URL construction updated to include /api
- content_publish.py docstring updated to reflect correct endpoint

Frontend:
- contentLinks.js: avoid double /api prefix in dev mode when backend already returns it
- stl.html: replace throw err with return after showing WebGL fallback message
- Rebuild dist

Tests:
- Update expected URLs in test_content_store.py and test_share_file.py

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Refactor: move tool helper modules into _internal subpackage ...

Moves non-tool infrastructure out of navi/tools/ root so that only
actual tool classes live there:

  base.py           → _internal/base.py
  loader.py         → _internal/loader.py
  middleware.py     → _internal/middleware.py
  logging_middleware.py → _internal/logging_middleware.py
  _time_parser.py   → _internal/time_parser.py

All imports updated across core/, api/, mcp/, tools/, and tests/.
No proxy files remain in navi/tools/ root.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Complete phase 7 regression test coverage

Clarify share file publishing boundaries