feat(uat-batch-9): milestone classifier + skip-ahead backfill controls

Two coordinated UX changes that finally make the rep's manual-stage-
jump workflow legible:

- Milestone phase classifier introduces a "stage-owning milestone"
  rule. When the rep manually advances the deal to Reservation+ but
  earlier sub-statuses are still un-signed, the current-stage
  milestone now stays marked `'current'` (no longer collapses into
  the past-strip / upcoming-accordion based on completion alone).
  Earlier-than-stage milestones bucket to `'past'` so the rep can
  backfill them; later slots stay `'future'`. The previous
  firstIncompleteKey-driven rule still applies in stages without an
  owning milestone (enquiry / qualified / nurturing).
- Skip-ahead backfill control `<MilestoneBackfillButton>` lands in
  the past-milestones strip whenever a milestone's date column is
  null. Opens a DatePicker popover (today default, accepts any past
  date) and PATCHes the relevant date_* column directly via
  useInterestPatch — no stage transition fires.
- `InterestPatchField` extended with the five milestone date keys;
  validator gains `dateDepositReceived` (was the only missing one).

Together this means: a deal manually-advanced from EOI Sent → Deposit
no longer hides Reservation under upcoming-milestones AND the rep can
record the EOI/reservation signing dates without re-triggering the
stage transition.

tsc clean. 1419/1419 vitest pass.

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

src/lib/validators/interests.ts

@@ -63,6 +63,7 @@ export const createInterestSchema = z.object({
   dateEoiSent: z.coerce.date().nullable().optional(),
   dateEoiSigned: z.coerce.date().nullable().optional(),
   dateReservationSigned: z.coerce.date().nullable().optional(),
+  dateDepositReceived: z.coerce.date().nullable().optional(),
   dateContractSent: z.coerce.date().nullable().optional(),
   dateContractSigned: z.coerce.date().nullable().optional(),
   pipelineStage: z.enum(PIPELINE_STAGES).default('enquiry'),