Migrate all components and pages to shadcn/ui with DataTable

Replace custom markup across all pages and components with shadcn/ui
primitives (Dialog, Sheet, Select, Card, Tabs, etc.). Add reusable
DataTable component powered by @tanstack/react-table with sortable
column headers and client-side pagination. Introduce TransactionList
with responsive mobile cards and desktop DataTable, dashboard section
customization (DashboardSection, SectionConfigDialog), and settings
API types.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api.ts

@@ -56,6 +56,34 @@ export interface ImportResult {
   errors: string[];
 }
 
+// --- User Settings ---
+
+export interface SectionSettings {
+  label: string;
+  color: string;
+  cardColor: string;
+  visible: boolean;
+  order: number;
+  expanded: boolean;
+}
+
+export interface DashboardSettings {
+  sections: Record<string, SectionSettings>;
+}
+
+export interface UserSettingsData {
+  dashboard: DashboardSettings;
+}
+
+export interface UserSettingsResponse {
+  key: string;
+  data: UserSettingsData;
+  updated_at: string;
+}
+
+export const getSettings = () => api.get<UserSettingsResponse>('/settings/');
+export const updateSettings = (data: UserSettingsData) => api.patch<UserSettingsResponse>('/settings/', { data });
+
 export interface Transaction {
   id: number;
   amount: number;