feat(manage): add /manage pages to edit caption and date

/manage lists every photo; /manage/edit/{slug} rewrites the photo's
.toml sidecar, which triggers a rebuild. The pages sit behind the same
htpasswd as /upload and are not linked from the public site.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

templates/edit.html

@@ -0,0 +1,110 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1.0">
+  <meta name="robots" content="noindex">
+  <title>Edit — {{.Photo.Caption}}</title>
+  <link rel="stylesheet" href="{{.FontsURL}}">
+  <link rel="stylesheet" href="/static/style.css">
+  <style>
+    :root { --serif: '{{.SerifFamily}}', Georgia, serif; --mj: '{{.MlFamily}}', sans-serif; }
+    .edit-page {
+      max-width: 520px;
+      margin: 0 auto;
+      padding: 32px 20px;
+    }
+    .edit-page .back-btn {
+      color: var(--fg3);
+      text-decoration: none;
+      font-size: 15px;
+    }
+    .edit-page .back-btn:hover { color: var(--fg); }
+    .edit-page h2 {
+      font-family: var(--serif);
+      font-size: 22px;
+      font-weight: 600;
+      color: var(--fg);
+      margin: 16px 0 24px;
+    }
+    .edit-img {
+      display: block;
+      width: 100%;
+      max-height: 340px;
+      object-fit: contain;
+      background: var(--bg0);
+      border: 1px solid var(--bg2);
+      margin-bottom: 8px;
+    }
+    .edit-slug {
+      font-size: 12px;
+      color: var(--fg4);
+      font-family: monospace;
+      margin-bottom: 24px;
+      overflow-wrap: anywhere;
+    }
+    .field { margin-bottom: 18px; }
+    .field label {
+      display: block;
+      font-size: 12px;
+      color: var(--fg4);
+      font-family: monospace;
+      text-transform: uppercase;
+      letter-spacing: 0.06em;
+      margin-bottom: 6px;
+    }
+    .field input[type="text"],
+    .field input[type="date"] {
+      width: 100%;
+      background: var(--bg0);
+      border: 1px solid var(--bg2);
+      color: var(--fg);
+      font-size: 16px;
+      padding: 10px 12px;
+      outline: none;
+      transition: border-color 0.2s;
+    }
+    .field input[type="text"]  { font-family: var(--serif); }
+    .field input[type="date"]  { font-family: monospace; font-size: 14px; color-scheme: dark; }
+    .field input:focus { border-color: var(--fg4); }
+    .submit-btn {
+      width: 100%;
+      background: var(--bg1);
+      border: 1px solid var(--bg2);
+      color: var(--fg2);
+      font-family: var(--serif);
+      font-size: 16px;
+      font-weight: 600;
+      padding: 12px;
+      cursor: pointer;
+      transition: background 0.2s, color 0.2s;
+    }
+    .submit-btn:hover { background: var(--bg2); color: var(--fg); }
+  </style>
+</head>
+<body>
+  <div class="edit-page">
+    <a href="/manage" class="back-btn">← Manage</a>
+    <h2>Edit photo</h2>
+
+    <img class="edit-img" src="/{{.Photo.Medium}}" alt="">
+    <p class="edit-slug">{{.Photo.Slug}}</p>
+
+    <!-- A plain form post, no script: the server answers with a redirect
+         back to /manage, so a reload never resubmits the edit. -->
+    <form method="post" action="/manage/edit/{{.Photo.Slug}}">
+      <div class="field">
+        <label for="caption">Caption</label>
+        <input type="text" id="caption" name="caption" value="{{.Photo.Caption}}" autocomplete="off" autofocus>
+      </div>
+
+      <div class="field">
+        <label for="date">Date</label>
+        <input type="date" id="date" name="date" value="{{.Photo.Date.Format "2006-01-02"}}" required>
+      </div>
+
+      <button class="submit-btn" type="submit">Save changes</button>
+    </form>
+  </div>
+</body>
+</html>