feat: complete user config management

- Add theme support with dark/light/system modes
- Add useTheme hook for applying user config theme
- Update router to use SettingsPage
- Update app-shell to apply theme on load
- Add CSS variables for dark theme
- Fix mypy errors in user_config.py
- Quality gates pass: ruff, mypy, typecheck, lint, build

openspec/changes/user-config-management/specs/user-config-management/spec.md

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+## ADDED Requirements
+
+### Requirement: User Config Storage
+
+The system SHALL store user configuration as JSONB key-value pairs linked to the user.
+
+#### Scenario: Create config on first write
+- GIVEN an authenticated user with no config
+- WHEN they update settings
+- THEN a UserConfig row is created with their user_id
+
+### Requirement: Config Keys
+
+The system SHALL support these configuration keys:
+- `default_editor`: string
+- `theme`: "light" | "dark" | "system"
+- `git_user_name`: string
+- `git_user_email`: string
+
+#### Scenario: Update theme
+- GIVEN an authenticated user
+- WHEN they PATCH /users/me/config with {"theme": "dark"}
+- THEN only the theme key is updated
+- AND other keys remain unchanged
+
+### Requirement: Config Retrieval
+
+The system SHALL return user configuration on request.
+
+#### Scenario: Get config
+- GIVEN an authenticated user with config
+- WHEN they GET /users/me/config
+- THEN their full configuration is returned
+
+### Requirement: Frontend Theme Application
+
+The system SHALL apply the user's theme preference on application load.
+
+#### Scenario: Load with dark theme
+- GIVEN a user with theme="dark"
+- WHEN the app loads
+- THEN the dark CSS class is applied to the document