Refactor settings system and improve context initialization

Backend:
- Add type validation and coercion for settings API
- Implement SettingStorage and SettingType in registry
- Improve CRUD operations for settings

Frontend:
- Refactor Theme, Language, Sidebar, ViewMode contexts
- Simplify admin components (GeneralTab, SettingsTab, UsersTab)
- Add new settings endpoints to API client
- Improve App initialization flow

Infrastructure:
- Update Dockerfile and docker-compose.yml
- Add .dockerignore
- Update Makefile and README

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,37 @@
+.git
+.gitignore
+
+# IDE / local tooling
+.vscode
+.idea
+.claude
+
+# Python
+__pycache__/
+*.py[cod]
+.venv
+venv/
+backend/venv/
+backend/__pycache__/
+backend/**/__pycache__/
+.pytest_cache/
+
+# Node / Vite
+frontend/node_modules/
+frontend/dist/
+frontend/.vite/
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+.pnpm-store/
+
+# Local persistent data (runtime bind-mounted)
+config/
+*.db
+*.sqlite
+*.sqlite3
+
+# OS
+.DS_Store
+Thumbs.db