release: v0.13.0 (#535)

Twenty-nine commits since v0.12.0. See CHANGELOG.md for the full list
and the upgrade notes.

Ten additions — the SEO package (site URL, robots.txt, sitemap.xml,
JSON-LD), slideshow mode, opt-in original downloads, copy-link,
previous/next album navigation, alt text from the Immich description,
per-album location precision, the update notice, a Help tab, and a
subpage cover grid separate from its photo grid. Two fixes: CJK album
slugs, and the lightbox control bar.

Photo of the Day (#476) was built on `dev` and reverted as not planned;
it never appears in a release.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01DmsHVRL9yQvckUB1ZpVWre

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: ralksta <ralksta@users.noreply.github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

lib/urls.ts

@@ -96,6 +96,37 @@ export function assetExifSummary(asset: Pick<ImmichAsset, 'exifInfo'>): ExifSumm
   return { camera, lens, focalLength };
 }
 
+/**
+ * The download URL for an original file (#475).
+ *
+ * Carries the album as well as the asset: the route authorises the download
+ * against the album that offered it, and checks the asset really belongs to
+ * that album.
+ */
+export function downloadUrl(albumId: string, assetId: string): string {
+  return `/api/download/${encodeAssetId(albumId)}/${encodeAssetId(assetId)}`;
+}
+
+/**
+ * The Immich asset description, trimmed, for use as image alt text.
+ *
+ * Gated on the `caption` EXIF group rather than served unconditionally. That
+ * switch exists because the description is the one field that can hold private
+ * notes (#506), and an alt attribute publishes it to crawlers and view-source
+ * exactly as a visible caption would. A site that has switched captions off has
+ * said it does not want that text on the page.
+ *
+ * Returns undefined when there is nothing to say, which leaves `alt=""` — the
+ * correct markup for a decorative image, and better than inventing filler.
+ */
+export function assetCaption(
+  asset: Pick<ImmichAsset, 'exifInfo'>,
+  showCaption: boolean,
+): string | undefined {
+  if (!showCaption) return undefined;
+  return asset.exifInfo?.description?.trim() || undefined;
+}
+
 /**
  * Compute the natural aspect ratio (width / height) from EXIF dimensions.
  * Returns undefined if dimensions are not available.