Reports frontend — spec-driven generate form, sliceable viewer, dimension bars (#16)

# MR: Reports frontend — spec-driven generate form, sliceable viewer, dimension bars

**Branch:** `cached-reporting` → **Target:** `main`

## Summary

Reports UI rebuilt to treat the backend OpenAPI spec as the source of truth: the
generate-report form renders generically from `ReportQuery`, and the viewer displays
API-computed metrics verbatim instead of recomputing them client-side.

## Changes

### react-openapi (library)
- `MultiEnumField` renderer for array-of-enum fields; dispatch branch in
  `FormFieldRenderer`
- `useFkFieldOptions` hook + `FKFieldConfig.value` (FK options keyed by a non-PK
  field — accounts resolve by `name`)
- `FieldConfig.defaultValue` seeding in `extractFields`; enum lift for
  `items.enum` arrays; sanitize-payload bypasses FK resolution when `fk.value` is set

### Reports page
- **GenerateReportPanel**: rewritten (~275 → ~110 lines) — fields extracted from the
  served `ReportQuery` schema (labels, order, defaults, multi-enum, account picker);
  server-side validation only
- **ReportViewer**:
  - Period groups built from cube buckets keyed by canonical `period_id`; buckets
    merged per period (disjoint slices), txn ids deduped
  - Cards/bars/metric strips read `metrics` verbatim (outflows/inflows/sum/count/
    avg/min/max/cadence/frequency); Net = one subtraction at render
  - Dimension-switchable bar strip (Period / Payees / Tags): segmented pill control
    with sliding indicator; pills disabled when the report has ≤1 distinct value;
    top-10 bars in a fixed view paged by chevrons (hidden scrollbar)
  - Active dim requests the full distribution via `payee=*` / `tags=*` when its own
    filter is unselected; explicit selections always slice normally
- **TransactionList**: shared component with optional `groups` prop (API periods +
  metrics) used by the viewer; Expense page keeps the client-side fallback path;
  group strip shows Sum · Avg · Min · Max · Cadence (or Frequency /day when
  cadence < 1)

### Fixes
- Strict DD-MM-YYYY parsing (`parseOccurredAt`) in period merges and date ordering —
  `new Date()` mis-read dates as MM-DD-YYYY and corrupted merged cadence
  (Aug Zomato: 12.89 days displayed vs 0.76 actual)
- Bar widths magnitude-based so all-outflow slices render at proper width

Reviewed-on: #16
Co-authored-by: Vishesh 'ironeagle' Bangotra <aetoskia@gmail.com>
Co-committed-by: Vishesh 'ironeagle' Bangotra <aetoskia@gmail.com>

react-openapi/src/components/fields/FormFieldRenderer.tsx

@@ -6,6 +6,7 @@ import { NumberField } from "./renderers/NumberField";
 import { DateField } from "./renderers/DateField";
 import { BooleanField } from "./renderers/BooleanField";
 import { EnumField } from "./renderers/EnumField";
+import { MultiEnumField } from "./renderers/MultiEnumField";
 import { FkSelectField } from "./renderers/FkSelectField";
 import { FkMultiSelectField } from "./renderers/FkMultiSelectField";
 import { FileUploadField } from "./renderers/FileUploadField";
@@ -81,6 +82,17 @@ export function FormFieldRenderer({ field, value, onChange, error, fkOptions, fk
     );
   }
 
+  if (field.isArray && field.enumValues) {
+    return (
+      <MultiEnumField
+        field={field}
+        value={value}
+        onChange={onChange}
+        error={error}
+      />
+    );
+  }
+
   if (field.enumValues) {
     return (
       <EnumField