Audit short-term: currency preview, date bounds, success destination, safe-area

- H3: Live currency preview below currency inputs shows the value formatted
  with thousands separators (en-US, USD) using Intl.NumberFormat. Skipped
  inside matrix cells to keep the Y1 monthly table compact.
- M1: Date inputs now apply min/max bounds. Default window is 1900-01-01 to
  2100-12-31; per-field override via FieldConfig.min/max as ISO strings.
- H6: On successful submit, replace the form with a SuccessDestination card
  (large checkmark, org name, "Submit another" + "safe to close" affordance).
  Prevents accidental duplicate submits from back-button / autofill replay.
- M6: Sticky submit bar respects iOS safe-area-inset-bottom.

FieldRenderer now takes a control prop so the currency preview can subscribe
to its single field via useWatch without re-rendering the whole form.

types/form.ts

@@ -69,9 +69,13 @@ export interface FieldConfig {
   placeholder?: string;
   /** For select / multiselect. */
   options?: SelectOption[];
-  /** For number / currency / percent — min/max/step constraints. */
-  min?: number;
-  max?: number;
+  /**
+   * Min/max bound. For numeric fields, a number; for date fields, an ISO
+   * date string (YYYY-MM-DD). Date fields default to a wide sanity window
+   * (1900-01-01 to 2100-12-31) when not set.
+   */
+  min?: number | string;
+  max?: number | string;
   step?: number;
   /** For text / textarea — max length. */
   maxLength?: number;