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/components/SideMenu.tsx

@@ -30,11 +30,6 @@ export function SideMenu({ resources, basePath, mobileOpen, onClose }: SideMenuP
   const theme = useTheme();
   const isMobile = useMediaQuery(theme.breakpoints.down("md"));
 
-  const colors = [
-    "#6366f1", "#10b981", "#f59e0b", "#ef4444", "#8b5cf6",
-    "#ec4899", "#14b8a6", "#f97316", "#06b6d4", "#84cc16",
-  ];
-
   const content = (
     <Box>
       <Toolbar>
@@ -43,7 +38,7 @@ export function SideMenu({ resources, basePath, mobileOpen, onClose }: SideMenuP
         </Typography>
       </Toolbar>
       <List sx={{ px: 1 }}>
-        {resources.map((r, i) => {
+        {resources.map((r) => {
           const listPath = `${basePath}/${r.name}`;
           const active = location.pathname.startsWith(listPath);
           return (
@@ -58,13 +53,15 @@ export function SideMenu({ resources, basePath, mobileOpen, onClose }: SideMenuP
                 borderRadius: 2,
                 mb: 0.5,
                 "&.Mui-selected": {
-                  bgcolor: `${colors[i % colors.length]}15`,
-                  "&:hover": { bgcolor: `${colors[i % colors.length]}20` },
+                  bgcolor: "primary.main",
+                  color: "primary.contrastText",
+                  "&:hover": { bgcolor: "primary.main" },
+                  "& .MuiListItemIcon-root": { color: "primary.contrastText" },
                 },
               }}
             >
               <ListItemIcon sx={{ minWidth: 36 }}>
-                <CircleIcon sx={{ color: colors[i % colors.length], fontSize: 12 }} />
+                <CircleIcon sx={{ fontSize: 12 }} />
               </ListItemIcon>
               <ListItemText
                 primary={r.displayName}