feat(email): convert single-column templates to shared partials + brand

docs/superpowers/plans/2026-06-26-email-templates-brand-hygiene.md

@@ -477,19 +477,19 @@ git commit -m "feat(email): add theme language file with preheader copy"
 | `invitation.html` | `invitation.preheader` | invite link (existing var, as present) |
 | `test.html` | `test.preheader` | none (skip button rows) |
 
-- [ ] **Step 1: Convert `welcome.html`** per the transform + its row above.
-- [ ] **Step 2: Convert `verify-email.html`.**
-- [ ] **Step 3: Convert `reset_notify.html`** (no button — keep only the text rows).
-- [ ] **Step 4: Convert `registration_accepted.html`.**
-- [ ] **Step 5: Convert `invitation.html`.**
-- [ ] **Step 6: Convert `test.html`** (no button).
+- [x] **Step 1: Convert `welcome.html`** per the transform + its row above.
+- [x] **Step 2: Convert `verify-email.html`.**
+- [x] **Step 3: Convert `reset_notify.html`** (no button — keep only the text rows).
+- [x] **Step 4: Convert `registration_accepted.html`.**
+- [x] **Step 5: Convert `invitation.html`.**
+- [x] **Step 6: Convert `test.html`** (no button).
 
-- [ ] **Step 7: Run the regression guard — expect only `banned`, `notification`, `digest` failing**
+- [x] **Step 7: Run the regression guard — expect only `banned`, `notification`, `digest` failing**
 
 Run: `node scripts/check-emails.js`
 Expected: FAIL listing exactly `banned.html`, `notification.html`, `digest.html`.
 
-- [ ] **Step 8: Build + commit**
+- [x] **Step 8: Build + commit**
 
 ```bash
 ./nodebb build