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>

go/internal/web/templates/partials/nav.tmpl

@@ -0,0 +1,17 @@
+{{define "nav"}}
+<div class="nav">
+    <div>
+        <a href="/adults"{{if eq .Active "adults"}} class="active"{{end}}>[Adults]</a>
+        <a href="/juniors"{{if eq .Active "juniors"}} class="active"{{end}}>[Juniors]</a>
+    </div>
+    <div class="nav-archived">
+        <span style="color: #666; margin-right: 5px;">Archived:</span>
+        <a href="/payments"{{if eq .Active "payments"}} class="active"{{end}}>[Payments Ledger]</a>
+    </div>
+    <div class="nav-archived">
+        <span style="color: #666; margin-right: 5px;">Tools:</span>
+        <a href="/sync-bank"{{if eq .Active "sync"}} class="active"{{end}}>[Sync Bank Data]</a>
+        <a href="/flush-cache"{{if eq .Active "flush"}} class="active"{{end}}>[Flush Cache]</a>
+    </div>
+</div>
+{{end}}