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/design.md

@@ -0,0 +1,32 @@
+## Context
+
+The platform needs a simple key-value configuration system for user preferences like theme, editor, and git identity.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Store user config as JSONB in PostgreSQL
+- Support partial updates (PATCH semantics)
+- Apply theme in frontend on load
+- Provide settings UI
+
+**Non-Goals:**
+- Complex nested config structures
+- Config validation beyond type checking
+- Per-project config (global only)
+
+## Decisions
+
+1. **JSONB column for flexibility**
+   - Rationale: Simple key-value, no schema migrations for new keys
+   
+2. **Merge semantics for updates**
+   - Rationale: Frontend can update single key without sending entire config
+   
+3. **Lazy creation**
+   - Rationale: Config row created on first write, not on user creation
+
+## Risks
+
+- **[JSONB query performance]** → Only querying by user_id, not by config keys
+- **[No schema validation]** → Frontend validates known keys, backend accepts any JSON