feat(go): M6.1 — template skeleton, embed.FS, HTML routes

Stand up the Go-native HTML frontend foundation:
- base.tmpl layout + nav/footer partials (three-tier nav, active-link highlighting)
- terminal-green-on-black theme extracted to static/css/app.css (served via embed.FS)
- HTMLHandler with stub pages for all five routes; / redirects to /adults
- NewRenderer parses per-page template sets at startup so parse failures abort boot
- Smoke test: each route returns 200 text/html with exactly one class="active" link

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Changelog
 
+## 2026-05-08 00:44 CEST — feat(go): M6.1 — template skeleton + embed.FS
+
+- `go/internal/web/templates/`: `base.tmpl` (full HTML layout), `partials/nav.tmpl` (three-tier nav with active-link highlighting), `partials/footer.tmpl` (build meta), and stub pages for each route (adults/juniors/payments/sync/flush_cache).
+- `go/internal/web/static/css/app.css`: terminal-green-on-black theme extracted once from Python `templates/adults.html` — shared by all Go HTML pages via `<link>`.
+- `go/internal/web/assets.go`: `//go:embed templates static` for single-binary deployment.
+- `go/internal/web/render.go`: `Renderer` parses a fresh `*template.Template` per page at startup; `Render(w, name, data)` executes the "base" template block.
+- `go/internal/web/html_handler.go`: `HTMLHandler` with one method per route (`ServeAdults`, `ServeJuniors`, `ServePayments`, `ServeSync`, `ServeFlushCache`).
+- `go/internal/web/server.go`: drops `helloHandler`; `GET /{$}` now redirects to `/adults`; HTML + `/static/` routes registered alongside the existing `/api/*` routes.
+- `go/internal/web/html_handler_test.go`: smoke test — each route returns 200 `text/html` with exactly one `class="active"` on the matching nav link.
+
 ## 2026-05-08 00:26 CEST — fix(py): parity coercions — amount/message types + junior '?' sticky
 
 - `scripts/match_payments.py`: added `get_float` helper — non-numeric `amount` values (e.g. `"---"` placeholder rows) now coerce to `0.0` matching Go's `parseFloat` behaviour; `message` field now goes through `get_str` so numeric cell values (bank references) are emitted as strings, matching Go's `fmt.Sprint`.