## 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
