feat: add customizable business branding (name, logo, colors)

Add admin settings for business branding with name, logo upload, and
color scheme via CSS custom properties. Includes database migration,
API endpoints, admin settings page, and dynamic branding in both
admin nav and customer portal.

Closes #61

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

packages/types/src/index.ts

@@ -145,6 +145,17 @@ export interface Invoice {
   tipSplits?: InvoiceTipSplit[];
 }
 
+export interface BusinessSettings {
+  id: string;
+  businessName: string;
+  logoBase64: string | null;
+  logoMimeType: string | null;
+  primaryColor: string;
+  accentColor: string;
+  createdAt: string;
+  updatedAt: string;
+}
+
 // Paginated list response
 export interface PaginatedList<T> {
   items: T[];