Add tenant branding & theming system with admin UI

Tenant admins can now customize their workspace appearance: page/nav
backgrounds (solid, gradient, or image), column/card colors, borders,
text, opacity with glass blur, card corner radius, and accent colors.
Light and dark mode are configured separately. Theme CSS is injected
server-side via transformPageChunk to prevent FOUC.

Includes avatar components, account page updates, and pre-existing
type error fixes (Buffer, async onMount, InputJsonValue).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/server/auth.ts

@@ -35,7 +35,8 @@ export async function validateSession(sessionId: string) {
 						name: true,
 						globalRole: true,
 						tenantRole: true,
-						tenantId: true
+						tenantId: true,
+						avatarUrl: true
 					}
 				}
 			}