docs(16-03): complete FieldPlacer signer selector plan

.planning/phases/16-multi-signer-ui/16-03-SUMMARY.md

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+---
+phase: 16-multi-signer-ui
+plan: "03"
+subsystem: ui
+tags: [react, typescript, fieldplacer, signer-coloring, active-signer]
+
+# Dependency graph
+requires:
+  - phase: 16-multi-signer-ui
+    plan: "01"
+    provides: signers prop + unassignedFieldIds prop threaded into FieldPlacer
+provides:
+  - activeSignerEmail state in FieldPlacer — auto-defaults to signers[0].email
+  - Active signer selector UI (select dropdown with colored dot, shown when signers.length > 0)
+  - Per-signer field coloring — dragged fields get signerEmail; color overrides type color when assigned
+  - Unassigned field red highlight — border/background #ef4444 when field.id in unassignedFieldIds
+  - DragOverlay ghost uses activeSigner.color
+
+affects: [16-04-dashboard-badge]
+
+---
+
+# Phase 16 Plan 03: FieldPlacer Active Signer Selector and Color Override Summary
+
+## What Was Built
+
+Added multi-signer UX to `FieldPlacer.tsx` (822 → 907 lines):
+
+1. **`activeSignerEmail` state** — `useState<string | null>(null)` initialized via `useEffect` to `signers[0].email` on mount; resets if signers prop changes or selected signer removed.
+
+2. **`handleDragEnd` signer auto-assignment** — `...(activeSignerEmail ? { signerEmail: activeSignerEmail } : {})` added to `newField` construction so dragged fields are tagged automatically.
+
+3. **`renderFields` signer color override** — `signers.find(s => s.email === field.signerEmail)?.color` overrides type-based color when the field has a signerEmail. Fields without signerEmail retain existing PALETTE_TOKENS color.
+
+4. **Unassigned field validation overlay** — `isUnassigned` flag from `unassignedFieldIds.has(field.id)` applies `border: 2px solid #ef4444` / `background: #ef444414` to highlight fields needing signer assignment.
+
+5. **Active signer selector UI** — Native `<select>` at `height: 32px` above field palette. Shows only when `!readOnly && signers.length > 0`. Colored dot indicator (8×8px circle in signer color). Label: "Active signer:" at 12px/500 weight.
+
+6. **DragOverlay ghost** — Uses `activeSigner.color` for the ghost field color when an active signer is selected.
+
+# Tech tracking
+tech-stack:
+  added: []
+  removed: []
+  changed: []
+
+# Metrics
+duration: 3min
+commit: d768fc6