feat(ui): Create content pattern components - DataTable, FilterBar, EmptyState, LoadingState

- Add LoadingState with table, card, list, and text skeleton patterns
- Add EmptyState with customizable icon, title, description, and action slot
- Add FilterBar with search input, clear button, and custom filter slot
- Add DataTable with TanStack Table integration, sorting, and row click
- Create barrel export index.ts for common components
- Install tanstack-table-8-svelte-5 for Svelte 5 compatibility
- Sync auth spec with authenticated user redirect requirements
- Archive p03-dashboard-enhancement

Refs: openspec/changes/p04-content-patterns
Closes: p04-content-patterns

openspec/changes/headroom-foundation/specs/authentication/spec.md

@@ -85,6 +85,23 @@ The system SHALL include user information in JWT token claims.
   - exp (expiration timestamp, 60 minutes from iat)
   - jti (unique token ID)
 
+### Requirement: Authenticated user redirect
+The system SHALL redirect authenticated users away from login page to dashboard.
+
+#### Scenario: Authenticated user accesses login page
+- **GIVEN** a user has valid access token in localStorage
+- **WHEN** the user navigates to /login
+- **THEN** the system detects the valid token
+- **AND** redirects the user to /dashboard
+- **AND** does not display the login form
+
+#### Scenario: Auth state persists after page refresh
+- **GIVEN** a user is logged in with valid tokens
+- **WHEN** the user refreshes the page
+- **THEN** the system reads tokens from localStorage
+- **AND** restores authentication state
+- **AND** displays the authenticated content (not blank page)
+
 ### Requirement: Refresh token storage
 The system SHALL store refresh tokens in Redis with TTL.