feat(ml): Settings → Tagging 'Crop proposers' card (#134 step 3)

Exposes the detector config (per-proposer enable + weights + confidence, caps,
dedupe IoU) in Settings → Tagging, backed by MLSettings via /api/ml/settings.
ml_admin adds the detector fields to _EDITABLE + GET payload + validation (conf
0..1, caps >=1, IoU 0..1). New CropProposersCard.vue (mirrors HeadsCard) with
working defaults pre-filled, per-field live-save (no restart — the agent picks
changes up on its next lease), weights-format help, switch-revert on error.
Closes milestone #134: all three proposers are on out-of-the-box and tunable in
the UI. Test: detector defaults GET + patch round-trip + range validation.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/components/settings/MaintenancePanel.vue

@@ -36,6 +36,7 @@
       </p>
       <div class="fc-tile-stack">
         <MLThresholdSliders />
+        <CropProposersCard />
         <HeadsCard />
         <AliasTable />
       </div>
@@ -75,6 +76,7 @@ import MissingFileRepairCard from './MissingFileRepairCard.vue'
 import GpuTriageCard from './GpuTriageCard.vue'
 import DbMaintenanceCard from './DbMaintenanceCard.vue'
 import MLThresholdSliders from './MLThresholdSliders.vue'
+import CropProposersCard from './CropProposersCard.vue'
 import HeadsCard from './HeadsCard.vue'
 import GpuAgentCard from './GpuAgentCard.vue'
 import AliasTable from './AliasTable.vue'