Phase 6b/7b: dashboard codes editor, decode events, human-readable briefs (AUD-115)

- New /codes dashboard page (behind the session login, linked in the top bar):
  lists every mapping by kind with Add, inline Edit+Save and Delete. Sam can put
  real teacher surnames in without a rebuild (FR-17).
- Decode timetable EVENTS through the same map as periods, so an ensemble reads
  'JAZZ ENSEMBLE - Ensemble Room - Mr KWO' instead of raw codes.
- teacherName splits comma- or space-separated codes ('HHL, OFG').
- SeedCodes is a one-time bootstrap while the table is empty, so a correction or
  a delete is never overwritten or restored.
- Tests: decode Label/Event, the /codes page (save, refuse, csrf, delete, auth),
  store re-seed behaviour. go test ./internal/... passes.

internal/web/templates/codes.html

@@ -0,0 +1,88 @@
+{{define "codes.html"}}<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <meta name="color-scheme" content="light dark">
+  <title>{{.Title}} — Compass / Teams</title>
+  <link rel="stylesheet" href="/static/tokens.css">
+  <link rel="stylesheet" href="/static/app.css">
+  <script src="/static/theme.js" defer></script>
+</head>
+<body class="app">
+  <nav class="nav-bar">
+    <a class="nav-bar__brand" href="/">
+      <span class="nav-bar__mark" aria-hidden="true"></span>
+      Compass / Teams
+    </a>
+    <div class="nav-bar__right">
+      <a class="button-utility" href="/">What is due</a>
+      <button class="button-utility" type="button" data-theme-toggle>Dark</button>
+      <span class="nav-bar__user">{{.Username}}</span>
+      <form method="post" action="/logout" class="nav-bar__logout">
+        <input type="hidden" name="csrf" value="{{.CSRF}}">
+        <button class="button-utility" type="submit">Sign out</button>
+      </form>
+    </div>
+  </nav>
+
+  <main class="container">
+    <header class="page-head">
+      <h1 class="heading-1">Codes</h1>
+      <p class="page-head__sub">Plain-English names for the Compass codes in the briefs.
+        A code that is not here renders as the raw code — never blank.</p>
+    </header>
+
+    {{if .ErrMsg}}<p class="flash flash--error" role="alert">{{.ErrMsg}}</p>{{end}}
+    {{if .Flash}}<p class="flash flash--{{.FlashLevel}}" role="status">{{.Flash}}</p>{{end}}
+
+    {{range .Groups}}
+    <section class="section" aria-labelledby="codes-{{.Kind}}-title">
+      <div class="section__head">
+        <h2 id="codes-{{.Kind}}-title" class="heading-2">{{.Label}}</h2>
+        <span class="category-pill">{{len .Codes}}</span>
+      </div>
+      <p class="page-head__sub">{{.Hint}}</p>
+
+      <form method="post" action="/codes" class="inline-form">
+        <input type="hidden" name="csrf" value="{{$.CSRF}}">
+        <input type="hidden" name="kind" value="{{.Kind}}">
+        <input class="text-input" type="text" name="code" placeholder="code" required>
+        <input class="text-input" type="text" name="name" placeholder="plain English" required>
+        <button class="button-primary" type="submit">Add</button>
+      </form>
+
+      {{if .Codes}}
+      <ul class="item-list">
+        {{range .Codes}}
+        <li class="item">
+          <div class="item__body">
+            <form method="post" action="/codes" class="inline-form">
+              <input type="hidden" name="csrf" value="{{$.CSRF}}">
+              <input type="hidden" name="kind" value="{{.Kind}}">
+              <input type="hidden" name="code" value="{{.Code}}">
+              <code class="item__subject">{{.Code}}</code>
+              <input class="text-input" type="text" name="name" value="{{.Name}}" required>
+              <button class="button-primary" type="submit">Save</button>
+            </form>
+          </div>
+          <form class="item__actions" method="post" action="/codes/delete"
+                onsubmit="return confirm('Remove this mapping? The code renders raw again.');">
+            <input type="hidden" name="csrf" value="{{$.CSRF}}">
+            <input type="hidden" name="kind" value="{{.Kind}}">
+            <input type="hidden" name="code" value="{{.Code}}">
+            <button class="button-utility button-utility--danger" type="submit">Delete</button>
+          </form>
+        </li>
+        {{end}}
+      </ul>
+      {{else}}
+      <p class="section__empty">No codes yet. Add one above.</p>
+      {{end}}
+    </section>
+    {{end}}
+  </main>
+
+  <footer class="app-foot">Family Home Lab · Compass / Teams</footer>
+</body>
+</html>{{end}}