Add albums: group photos by free-text tags

An album is a tag. Photos carry lowercase free-text tags in their .toml
sidecar (`tags = [...]`), parallel to the existing day grouping — no
database, all derived at build time.

- albums.go: parseTags (lowercase/trim/dedupe, slug via sanitiseSlug),
  groupByTag mirroring groupByDay, and rendering for album pages, the
  index, and the JSON API. Slug collisions between distinct names are
  suffixed and logged.
- Routes: /album/<slug>/ (paginated square grid per tag), /albums/ (index
  with counts), /tags.json (machine-readable tag list, served statically
  like feed.xml).
- POST /upload gains a `tags` field written into the sidecar; the web
  upload form gains a tags input.
- Tags shown on each photo page linking to their album; an Albums link in
  the timeline/grid header.

templates/albums.html

@@ -0,0 +1,54 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1.0">
+  <title>Albums · Photo Gallery</title>
+  <link rel="stylesheet" href="{{.FontsURL}}">
+  <link rel="stylesheet" href="/static/style.css">
+  <link rel="icon" href="/static/favicon.ico" sizes="any">
+  <link rel="icon" type="image/png" href="/static/web-32.png" sizes="32x32">
+  <link rel="apple-touch-icon" href="/static/web-192.png">
+  <style>:root { --serif: '{{.SerifFamily}}', Georgia, serif; --mj: '{{.MlFamily}}', sans-serif; }</style>
+  <link rel="alternate" type="application/atom+xml" href="/feed.xml" title="Photo Feed">
+</head>
+<body>
+  <header class="profile-hdr">
+    <div class="profile-inner">
+      <div class="avatar-ring">
+        <span class="avatar-initials">{{.Initial}}</span>
+      </div>
+      <div class="pinfo">
+        <h1>{{.Author}}</h1>
+        <p class="handle">{{.Handle}}</p>
+        <p class="pcount"><strong>{{.Total}}</strong> posts</p>
+        <p class="pbio">{{.Bio}}{{if .BioAlt}}<br><span class="ml">{{.BioAlt}}</span>{{end}}</p>
+        <a href="/" class="rss-a">← Timeline</a>
+      </div>
+    </div>
+  </header>
+
+  <main>
+    <div class="grid-wrap">
+      <section class="tl-day">
+        <h2 class="tl-date">Albums<span class="tl-count">{{len .Albums}}</span></h2>
+        {{if .Albums}}
+        <div class="album-list">
+          {{range .Albums}}
+          <a href="/album/{{.Tag.Slug}}/" class="album-chip">
+            {{.Tag.Name}}<span class="album-chip-count">{{.Count}}</span>
+          </a>
+          {{end}}
+        </div>
+        {{else}}
+        <p class="album-empty">No albums yet — add tags to a photo to create one.</p>
+        {{end}}
+      </section>
+    </div>
+  </main>
+
+  <footer class="site-footer">
+    © 2000–{{.Year}} {{.Author}}. All rights reserved.
+  </footer>
+</body>
+</html>