feat(b3-1): interest dimensions dual-source — yacht dims for the recommender

Per docs/superpowers/audits/alpha-uat-master.md Bucket 3 #1. When a
yacht is linked to the interest the rep can flip a per-interest toggle
so the berth recommender reads dimensions off the yacht record instead
of the rep-entered desired_* columns.

- Migration 0087 + interests.useYachtDimensions boolean (default false).
- Validator (createInterestSchema) accepts the new field; service insert
  + update paths spread it through automatically.
- berth-recommender.service.loadInterestInput dual-source resolution:
  when toggle=true AND yachtId is set AND the yacht has at least one
  measurement on file, the recommender uses the yacht's length / width /
  draft instead of the desired_* values. Falls back to the desired
  columns whenever any precondition fails (no yacht link, toggle off,
  or the yacht carries no measurements). Returned InterestInput gains
  a `dimensionsSource: 'interest' | 'yacht'` trace field.
- Interest form: under the "Berth size desired" section, when a yacht
  is linked, a checkbox surfaces — "Use the linked yacht's dimensions
  for the recommender". When checked, the three dimension inputs grey
  out (DimensionInput gains a `disabled` prop) so the rep can't
  accidentally edit the now-overridden values. Hint text spells out
  the fallback behaviour.

Verified: tsc clean, 1493/1493 vitest, migration applied.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/interests/interest-form.tsx

@@ -860,6 +860,9 @@ interface DimensionInputProps {
   unit: 'ft' | 'm';
   ftValue: string | number | undefined;
   onChangeFt: (next: string | undefined) => void;
+  /** B3 #1: greyed out when the interest is set to read dimensions
+   *  off the linked yacht instead of these fields. */
+  disabled?: boolean;
 }
 
 /**
@@ -878,6 +881,7 @@ function DimensionInput({
   unit,
   ftValue,
   onChangeFt,
+  disabled,
 }: DimensionInputProps) {
   const focusedRef = useRef(false);
   const [display, setDisplay] = useState<string>(() => computeDisplay(ftValue, unit));
@@ -904,6 +908,7 @@ function DimensionInput({
         min={0}
         placeholder={placeholder}
         value={display}
+        disabled={disabled}
         onFocus={() => {
           focusedRef.current = true;
         }}