feat(email): add theme language file with preheader copy

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

@@ -395,7 +395,7 @@ git commit -m "test(email): structural regression guard for partial imports + pr
 **Interfaces:**
 - Produces: the `[[email:<name>.preheader]]` keys referenced by every template's preheader. Tasks 3, 6–9 reference these.
 
-- [ ] **Step 1: Create the language file with one preheader key per template**
+- [x] **Step 1: Create the language file with one preheader key per template**
 
 ```json
 {
@@ -412,7 +412,7 @@ git commit -m "test(email): structural regression guard for partial imports + pr
 }
 ```
 
-- [ ] **Step 2: Register the languages directory in `plugin.json`**
+- [x] **Step 2: Register the languages directory in `plugin.json`**
 
 Add a `"languages"` key alongside the existing `"templates": "templates"` key:
 
@@ -426,7 +426,9 @@ Add a `"languages"` key alongside the existing `"templates": "templates"` key:
 Run: `./nodebb build` then re-send the `reset` test email.
 Expected: build clean; the inbox preview shows "Reset your Shadows Over Westgate password", not the raw key.
 
-- [ ] **Step 4: Commit**
+Task 5 note: not run from this theme repo; there is no `./nodebb` executable here. JSON syntax was validated locally and `node scripts/check-emails.js` was run.
+
+- [x] **Step 4: Commit**
 
 ```bash
 git add languages/en-GB/email.json plugin.json