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.

static/style.css

@@ -422,3 +422,57 @@ body {
   .dlayout        { flex-direction: column; }
   .dsidebar       { width: 100%; border-left: none; border-top: 1px solid var(--line); }
 }
+
+/* ── Tags / albums ─────────────────────────────────────────── */
+
+.dtags {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 6px;
+  margin: 10px 0 14px;
+}
+
+.tag,
+.album-chip {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  padding: 3px 10px;
+  background: var(--bg0);
+  border: 1px solid var(--bg2);
+  color: var(--fg3);
+  font-family: monospace;
+  font-size: 12px;
+  text-decoration: none;
+  border-radius: 2px;
+  transition: color 0.15s, border-color 0.15s;
+}
+
+.tag:hover,
+.album-chip:hover {
+  color: var(--aqua);
+  border-color: var(--fg4);
+}
+
+.album-list {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 10px;
+  margin-top: 16px;
+}
+
+.album-chip {
+  font-size: 14px;
+  padding: 7px 14px;
+}
+
+.album-chip-count {
+  color: var(--fg4);
+  font-size: 11px;
+}
+
+.album-empty {
+  color: var(--fg4);
+  font-family: var(--serif);
+  margin-top: 16px;
+}