Stripe-style UI overhaul + Expenses feed with month grouping (#15)

## Title
Stripe-style UI overhaul + Expenses feed with month grouping

## Summary
Rework the khata-ui frontend to a Stripe-grade design system and add a
rich, monthly-grouped Expenses experience driven by react-openapi's
shared, spec-configured field renderers.

## Highlights

### Design overhaul
- New design system: single indigo (#635BFF), 6px radius, Inter type
  scale, streamlined surfaces/inputs/feedback tokens
- Sticky app bar with brand-left header + inline footer, route titles
- Split-panel auth pages, Stripe-style Home hero + feature cards
- Toast provider + shared PageHeader/EmptyState UI primitives
- Admin polish: right-aligned numerics, row-hover actions, skeletons,
  richer empty states, breadcrumbed fetch-request flows

### Expenses page
- Monthly grouped feed with stat cards (total spent / this month / income)
- Month cards as single-open accordions with red-spent / green-income totals
- Flat transaction rows: logo, name/date, account, amount (no accordion)
- Floating month–year selector pill (scroll-spy) that expands + scrolls
  to the selected month
- Strict DD-MM-YYYY date parsing (day-first), no format guessing

### Shared react-openapi fields
- New `CurrencyField` + `formatCurrency` (cached Intl, sign-colored,
  sign-less amounts; form editing reuses NumberField)
- `resourceConfig.fieldTypes` override mechanism (amount → currency)
- Expenses page rendered via ListCellRenderer / DetailFieldRenderer /
  applyDisplayFormat — zero custom field renderers
- `resolveMediaUrl` resolves relative `/uploads/...` against the API
  base for every image field (Admin + Expenses logos)

### Fixes
- Perf: memoized rows, formatter caches, single-open accordions —
  cuts slow-click INP and DOM-nesting warnings
- Removed unused @mui/x-data-grid@7, which hoisted @mui/system@7 and
  crashed Box/createTheme at runtime (v5/v7 mix)

## Commits (10)
- 9808a1f design overhaul + expenses page
- d6856a5 fix expenses list performance + dom nesting
- 3dd833a shared currency field + expenses rendered from react-openapi fields
- e8c585b fix expenses grouping to parse occurred_at as DD-MM-YYYY strictly
- cc940ee expense list: split month totals, sticky headers + month pill
- 47d799f expense list: clickable month pill + fix scroll-to-month
- 537aef9 resolve relative media URLs against the API base in react-openapi
- 8e9a13d expense list: replace accordion with flat Stripe-style rows
- 62dd06c expense list: month accordions + month-year selector; drop unused x-data-grid
- 806bd42 dropped amount sign from CurrencyField.tsx

Reviewed-on: #15
Co-authored-by: Vishesh 'ironeagle' Bangotra <aetoskia@gmail.com>
Co-committed-by: Vishesh 'ironeagle' Bangotra <aetoskia@gmail.com>

react-openapi/src/context/AppProvider.tsx

@@ -43,6 +43,20 @@ function extractProfileOperations(spec: any): ProfileOperation[] {
   return ops;
 }
 
+function applyResourceOverrides(configs: ResourceConfig[], specConfiguration: SpecConfiguration): ResourceConfig[] {
+  for (const resource of configs) {
+    const fieldTypes = specConfiguration.resourceConfig?.[resource.name]?.fieldTypes;
+    if (!fieldTypes) continue;
+    // fields and orderedFields share the same FieldConfig object references,
+    // so an in-place mutation updates both.
+    for (const field of resource.fields) {
+      const override = fieldTypes[field.name];
+      if (override) field.uiType = override;
+    }
+  }
+  return configs;
+}
+
 const DEFAULT_AUTH_CONFIG: AuthConfig = {
   serverUrl: "",
   loginPath: "/login",
@@ -88,7 +102,7 @@ export function AppProvider({ specConfiguration, children }: AppProviderProps) {
         if (errs.length === 0) {
           const configs = buildResourceConfigs(spec);
           if (!cancelled) {
-            setResources(configs);
+            setResources(applyResourceOverrides(configs, specConfiguration));
           }
 
           const baseUrl = specConfiguration.baseApiUrl ?? spec.servers?.[0]?.url ?? "";