feat(IMP-11): D-2 — frame min_height_px hint (backend → UI)

Step 9 v4_all_judgments[] now exposes per-candidate min_height_px from
catalog frame_contracts.visual_hints.min_height_px (None when contract
unregistered). SlideCanvas pendingLayout zones render a red ring + 'min H
Npx' badge when zone height falls below the active frame's threshold.
Visual hint only; resize clamp (minSize=0.05) unchanged.

5 axes (single commit per Stage 5 plan):
- u1 backend: src/phase_z2_pipeline.py — Step 9 builder adds min_height_px
  via single get_contract(c.template_id) lookup; reuses _contract for
  catalog_registered (no double-lookup).
- u2 type: Front/client/src/types/designAgent.ts — FrameCandidate gains
  optional minHeightPx?: number.
- u3 mapper: Front/client/src/services/designAgentApi.ts — maps snake-case
  min_height_px → camelCase minHeightPx on v4_all_judgments path;
  v4_candidates fallback remains undefined (graceful).
- u4 active-frame lookup: Front/client/src/components/SlideCanvas.tsx —
  activeFrameId = overrideFrameId ?? defaultFrameId; activeCandidate via
  region.frame_candidates.find.
- u5 hint render: Front/client/src/components/SlideCanvas.tsx —
  zoneHeightPx = height * SLIDE_H (logical px, no double-apply); compare
  against activeCandidate.minHeightPx in pendingLayout mode only; red
  border + badge when below.

Tests: 5/5 pass in tests/test_phase_z2_step9_v4_all_judgments_min_height.py
(source-string + catalog-shape guards + None propagation, registered and
unregistered template_ids).

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

Front/client/src/services/designAgentApi.ts

@@ -527,6 +527,10 @@ export async function loadRun(runId: string): Promise<LoadRunResult> {
         // backend step09 의 catalog_registered (frame_contracts.yaml 등록 여부).
         // v4_all_judgments 에만 있음. v4_candidates fallback 시 undefined.
         catalogRegistered: c.catalog_registered,
+        // backend step09 의 min_height_px (frame_contracts.yaml visual_hints.min_height_px).
+        // logical 1280x720 px 좌표계. contract 미등록 또는 visual_hints 부재 시 undefined.
+        // v4_all_judgments 에만 있음. v4_candidates fallback 시 undefined (graceful).
+        minHeightPx: c.min_height_px ?? undefined,
       }));
 
     const displayStrategy = (