docs(16-02): complete PreparePanel signer list plan

.planning/phases/16-multi-signer-ui/16-02-SUMMARY.md

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+---
+phase: 16-multi-signer-ui
+plan: "02"
+subsystem: ui
+tags: [react, typescript, preparepanel, signer-list, send-block-validation]
+
+# Dependency graph
+requires:
+  - phase: 16-multi-signer-ui
+    plan: "01"
+    provides: signers prop, onSignersChange, onUnassignedFieldIdsChange threaded from DocumentPageClient
+provides:
+  - SIGNER_COLORS palette constant ['#6366f1', '#f43f5e', '#10b981', '#f59e0b']
+  - Signer list UI in PreparePanel (email input + "Add Signer" button + colored dot rows + aria-labeled × remove)
+  - handleAddSigner / handleRemoveSigner with duplicate/format validation
+  - Send-block validation: fetch fields → check no-signers guard → check unassigned isClientVisibleField fields
+  - Inline error messages: "Add at least one signer..." and "{N} field(s) need a signer assigned..."
+  - signers persisted to documents.signers via prepare POST body
+
+affects: [16-04-dashboard-badge]
+
+---
+
+# Phase 16 Plan 02: PreparePanel Signer List UI Summary
+
+## What Was Built
+
+Modified `PreparePanel.tsx` (+104 lines) and `prepare/route.ts` (+5 lines):
+
+**PreparePanel.tsx:**
+- `SIGNER_COLORS = ['#6366f1', '#f43f5e', '#10b981', '#f59e0b']` — auto-assigned by index
+- Props destructured: `signers`, `onSignersChange`, `onUnassignedFieldIdsChange`
+- `handleAddSigner` — validates email format + duplicate, assigns next color from palette, calls `onSignersChange`
+- `handleRemoveSigner` — filters signer from list, clears `unassignedFieldIds`
+- Signer list UI section (Draft-only, inserted above Prepare and Send):
+  - Email `<input>` + "Add Signer" `<button>` (height 32px)
+  - Per-signer row: 8×8px colored dot (`w-2 h-2 rounded-full`) + email text + `×` remove button with `aria-label="Remove signer {email}"` (32×32px touch target)
+  - Empty state: "No signers added yet."
+  - Duplicate email warning: "That email is already in the signer list."
+- `handlePrepare` extended: fetches current fields via `/api/documents/{id}/fields`, checks unassigned `isClientVisibleField` fields, sets `unassignedFieldIds` and shows inline error if found
+- `signers` included in prepare POST body so `documents.signers` is persisted before send route reads it
+
+**prepare/route.ts:**
+- Body type extended with `signers?: DocumentSigner[]`
+- DB update spreads `{ signers: body.signers }` when provided
+
+# Tech tracking
+tech-stack:
+  added: []
+  removed: []
+  changed: []
+
+# Metrics
+duration: 4min
+commit: 1c8551c