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/proposal.md

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+## Why
+
+Users need to store preferences and settings (theme, editor, git identity) that persist across sessions.
+
+## What Changes
+
+- Add UserConfig model for JSONB key-value storage
+- Add API endpoints for get/update user config
+- Add frontend settings page
+- Apply theme preference in frontend
+
+## Capabilities
+
+### New Capabilities
+- `user-config-management`: Store and manage user preferences
+
+### Modified Capabilities
+- `user-profile`: Include config in profile responses
+
+## Impact
+
+- New database model and migration
+- New API endpoints under /users/me/config
+- New frontend settings page
+- Theme application in AppShell