Build stamp, live listening strip, OG cards, closer shiki ground

- footer: 'built <ts> · <hash> · <n> notes', hash linking to Gitea
  (GARDEN_COMMIT passed in by build.sh; '+' marks dirty-tree builds)
- homepage header: live strip fed by api.c0smere.net/spotify/summary
  (weekly aggregate + monthly top artist/genre), fail-closed like the
  odometer and header quote
- /og/<slug>.png satori+resvg cards for every note + the homepage;
  og:/twitter: meta in Base
- shiki github-dark-default (#0d1117 sits closer to the page ground)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/build-info.mjs

@@ -0,0 +1,35 @@
+// Build provenance for the footer stamp. Evaluated once per build (module
+// scope). The containerized build has no git binary, so deploy/build.sh
+// passes GARDEN_COMMIT in; local builds fall back to asking git directly.
+// A trailing "+" marks a build from a dirty tree.
+import { execSync } from 'node:child_process';
+
+function detectCommit() {
+  if (process.env.GARDEN_COMMIT) return process.env.GARDEN_COMMIT;
+  try {
+    const h = execSync('git rev-parse --short HEAD', {
+      stdio: ['ignore', 'pipe', 'ignore'],
+    })
+      .toString()
+      .trim();
+    let dirty = '';
+    try {
+      execSync('git diff --quiet', { stdio: 'ignore' });
+    } catch {
+      dirty = '+';
+    }
+    return h + dirty;
+  } catch {
+    return '';
+  }
+}
+
+export const COMMIT = detectCommit();
+
+const fmt = new Intl.DateTimeFormat('en-CA', {
+  dateStyle: 'short',
+  timeStyle: 'short',
+  hour12: false,
+  timeZone: 'America/New_York',
+});
+export const BUILT_AT = fmt.format(new Date()).replace(', ', ' ');