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/lib/validators/interests.ts

@@ -84,6 +84,11 @@ export const createInterestSchema = z.object({
   desiredLengthUnit: desiredUnitSchema,
   desiredWidthUnit: desiredUnitSchema,
   desiredDraftUnit: desiredUnitSchema,
+  /** Toggle: when true and a yacht is linked, the berth recommender
+   *  reads the yacht's dimensions instead of the desired_* columns
+   *  above. Per migration 0087. Defaults false everywhere it isn't
+   *  explicitly set. */
+  useYachtDimensions: z.boolean().optional(),
 });
 
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