feat(go): M6.7 — single-binary embed verification

Add TestEmbedCompleteness and TestStaticAssetsServed in
go/internal/web/assets_test.go. The completeness guard walks the
on-disk templates/ and static/ directories and asserts every file is
present in the corresponding embed.FS, catching forgotten files on
future additions. The static mux test hits /static/css/app.css and all
JS files through the same http.FileServerFS wiring used in server.go,
confirming assets are served from the embedded FS with correct
Content-Type and a 404 for unknown paths.

Standalone binary smoke test passed manually: binary copied to /tmp
(no adjacent templates/ or static/), assets served correctly.

Closes M6.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md

@@ -117,7 +117,7 @@ Goal: feature-equivalent UX on the Go side, designed cleanly. Not a Jinja port.
 - [x] **M6.5** Modal JS module (`static/js/member-detail.js`): fetches `/api/adults` (or juniors), renders status/exceptions/transactions on row click; keyboard nav (Esc, ↑/↓) — `e53e238`
 - [x] **M6.6** `/qr`, `/sync-bank`, `/flush-cache`, `/version` pages — `f6ba85b`
 - [x] **M6.6.1** Pay-button QR popup modal (`payment-qr.js`); restores Python `showPayQR` UX lost in M6.6 — `4276d7b`
-- [ ] **M6.7** Wire `embed.FS` into handlers; verify single-binary deployment includes all assets
+- [ ] **M6.7** Wire `embed.FS` into handlers; verify single-binary deployment includes all assets — (pending merge)
 
 **Gate:** Browser smoke on :8080: all pages render, name+month filters work, modal opens with correct data, QR loads, sync/flush work end-to-end.