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>

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## 2026-05-08 15:24 CEST — feat(go): M6.7 — single-binary embed verification
+
+- Confirmed `embed.FS` wiring is complete: templates parsed via `template.ParseFS(templateFS, ...)`, static assets served via `http.FileServerFS(fs.Sub(staticFS, "static"))`.
+- Added `go/internal/web/assets_test.go` with two tests: `TestEmbedCompleteness` (walks disk vs embed.FS to catch forgotten files) and `TestStaticAssetsServed` (hits `/static/css/app.css` and all JS files through the mux, asserts 200 + Content-Type + non-empty body + 404 for unknown paths).
+- Closes M6; single binary confirmed self-contained with no adjacent `templates/` or `static/` required at runtime.
+- Key files: `go/internal/web/assets_test.go` (new).
+
 ## 2026-05-08 14:55 CEST — feat(go): M6.6.1 — Pay-button QR popup modal
 
 - Restored the Python `showPayQR` in-page modal UX that was lost in M6.6 (Pay buttons were navigating the tab to the raw `/qr` PNG).