Update deployment docs — PostgreSQL-only, new .env vars, pg_trgm notes ...

- README.md: remove SQLite, add PostgreSQL setup (Ubuntu/Arch/Docker),
  add pg_trgm notes, update .env example
- docs/config.md: remove DB_PATH, add OLLAMA_REQUEST_TIMEOUT / OPENAI_MODEL /
  OPENAI_BASE_URL, update example .env
- .env.example: rewrite with all current variables and comments

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

WelcomeScreen polish, root-path fix, docs update ...

- Move mobile sidebar button to top-left corner (no header bar backdrop)
- Show WelcomeScreen on / with no hash instead of auto-loading first session
- Docs: document Ollama multi-server fallback, model priority lists, OLLAMA_BACKENDS_FILE

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

Make server I/O non-blocking; update docs ...

- Wrap all heavy filesystem ops in asyncio.to_thread: filesystem tool
  (read/write/append/list/find/info/move/delete/query/smart_edit),
  image_view (read_bytes), share_file (shutil.copy2), write_tool
  (write_text, _register_user_tool), session_files (shutil.rmtree,
  iterdir), sessions upload endpoint (sync open/write → to_thread)
- Make delete_session_dir async; update its caller in sessions.py
- docs/config.md: fix wrong defaults (threshold 0.70, keep_recent 8),
  remove phantom SESSION_FILES_TTL_HOURS, add LLM timeouts, DATABASE_URL,
  PUBLIC_URL, Gmail, CONTEXT_SUMMARY_MAX_TOKENS sections
- docs/profiles.md: add missing tool_developer profile to table
- android-client: add WebView remote debugging; remove unused toolbar menu
- Remove stale helper scripts and test files

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

Use gemma4 cloud model by default

Support Ollama Cloud API key

Add backend documentation ...

10 markdown files covering all backend subsystems: architecture,
agent loop, tool system, sessions, WebSocket protocol, profiles,
memory, config, and API reference. Structured for AI readability
with a single entry point (docs/index.md) and cross-references.

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