ALL-1875 PR-5: Merge comparison UI + ContactDetailPage merge history + Find Duplicates action

## ALL-1875: Contact merge UI + auto duplicate detection — PR 5 of 5 ### Goal Build the side-by-side merge comparison view, wire the mergeCanonicalContacts mutation into a confirmation flow, and add merge history + Find Duplicates action to ContactDetailPage. ### Context - Linear issue: ALL-1875 - Branch: `wilbo/all-1875-merge-ui` - Worktree: `mono-all-1875-merge-ui` - Depends on: PR-3 merged (mergeCanonicalContacts mutation + mergeHistory field available in GQL) - App: `apps/dashboard` ### What to build 1. **Merge comparison page route**: `apps/dashboard/app/(shell)/ws/[id]/contacts/[contactId]/merge/[otherId]/page.tsx` - Wraps `<ContactMergePage />` 2. **ContactMergePage component**: `apps/dashboard/components/pages/ContactMergePage.tsx` - Fetches both contacts by ID - Side-by-side field comparison table: name, email(s), phone(s), address, account numbers - Show match signals highlighted (e.g. email glow when both contacts have the same email) - Show associated resources counts: sites, devices, program enrollments, ThirdPartyClaims for each - "Merge" button: opens a confirmation dialog summarizing what will be merged - On confirm: calls `mergeCanonicalContacts(winnerId, loserId, reason: "Manual merge")` mutation - On success: redirect to winner contact detail page with a success toast - On error: surface the CanonicalContactMergeError code as a user-friendly message - Winner/loser selection: allow the user to pick which contact to keep (default to contactId from the URL) 3. **Merge history section on ContactDetailPage**: - Add a "Merge History" section at the bottom of ContactDetailPage.tsx - Query `canonicalContact { mergeHistory { ... } }` (from PR-3) - Shows a list of past merges: date, which contact was merged in, actor - "Undo merge" button for each (calls `unmergeCanonicalContacts` if exposed — if mutation doesn't exist yet, render a disabled button with tooltip "Contact support to undo") 4. **Find Duplicates action on ContactDetailPage**: - Add to the ActionMenu (already exists in ContactDetailPage.tsx) - Navigates to `/ws/[id]/contacts/duplicates?contactId=[id]` (pre-filters the list to pairs involving this contact) OR directly to the merge page if a single high-confidence match exists ### Key existing files - `apps/dashboard/components/pages/ContactDetailPage.tsx` — ActionMenu, Section, Card, SiteCard patterns; where to add merge history + action - `apps/dashboard/components/pages/ContactsPage.tsx` — list + filter patterns - `apps/dashboard/app/(shell)/ws/[id]/contacts/[contactId]/page.tsx` — existing route pattern - Mutation `mergeCanonicalContacts` already in GQL schema — find it in `apps/dashboard/lib/@generated/graphql.ts` ### Acceptance test - /ws/[id]/contacts/[id]/merge/[otherId] renders both contacts side-by-side - Confirming merge calls `mergeCanonicalContacts` mutation and redirects to winner - Merge history section on ContactDetailPage shows previous merges for a merged contact - Find Duplicates action in ActionMenu navigates correctly - TypeScript type-checks cleanly; no new TS errors ### Depends on - PR-3 merged (fleet-task e53c7925-185a-486b-a817-827d49985451) - PR-4 recommended (for the duplicates list navigation) but not hard-required ### Out of scope - Unmerge mutation implementation (follow-up Linear if mergeHistory.unmerge is needed — note the backend already has CanonicalContactMergeClaimSnapshot for it) - Fuzzy name match highlighting (nice-to-have; not blocking) ### Follow-up Linear issues to create after this PR ships - Unmerge UI (backend mutation + frontend undo button) — separate ticket - Workspace-level merge stats page — separate ticket