experimental: portfolio features (justified layout, unlisted pages, captions, per-album grid, focal point, nav links, cover hero) (#431)

## Was ist das?

Sieben neue Portfolio-Features, alle als **experimental** gekennzeichnet
— in YAML-Kommentaren, Admin-Panel-Labels und Commit-Scopes, damit sie
einzeln nachvollziehbar und revertierbar bleiben.

## Features (je ein Commit)

| Commit | Feature |
| --- | --- |
| `justified rows gallery layout` | Neues Grid-Layout `justified`:
zeilenbasiert, gleiche Höhen, volle Breite — Flexbox ohne JS-Messung,
letzte Zeile bleibt natürlich klein |
| `hide subpages from navigation` | `hidden: true`: Seite aus
Nav/Homepage raus, per Direktlink erreichbar — im Gegensatz zu `enabled:
false` (404) |
| `lightbox captions` | Immich-Asset-Description als Bildunterschrift im
Info-Panel; EXIF-Route liefert bei `exifOnHover: false` jetzt nur noch
die Caption statt 403 |
| `per-album grid overrides` | `grid:` pro Album-Eintrag, Präzedenz
global < Subpage < Album |
| `per-album cover focal point` | `coverPosition:` → `object-position`
auf Cover-Karten, strikt validiert |
| `external links in header navigation` | `navLinks:` in settings.yaml,
nur http(s), sanitisiert |
| `cover splash hero style` | `heroStyle: cover`: Fullscreen-Splash mit
Enter-Link ins Portfolio |

Alle Optionen sind im Admin-Panel editierbar (PageBuilder /
SettingsEditor).

## Tests

- 356 Unit-Tests grün (13 neue: deriveGallery hidden/grid/coverPosition,
sanitizeNavLinks, EXIF-Route-Captions, justified-Layout-Validierung)
- `npx tsc --noEmit`: 0 Fehler
- `npm run build`: sauber
- Justified-Zeilenmathematik und Cover-Splash per Browser-Harness
visuell/numerisch verifiziert (volle Zeilen enden exakt an der
Containerkante, letzte Zeile bleibt natürlich)

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: ralksta <ralksta@users.noreply.github.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: lancetm714 <108419425+lancetm714@users.noreply.github.com>

app/[...path]/page.tsx

@@ -182,6 +182,17 @@ export default async function PathPage({ params, searchParams }: PathPageProps)
   const resolveLayout = (overrides?: Partial<GridConfig>) =>
     overrides?.layout ?? config.grid.layout;
 
+  // EXPERIMENTAL: per-album grid override — merged over the subpage grid so
+  // the precedence is global < subpage < album.
+  const mergeAlbumGrid = (
+    albumId: string,
+    spGrid?: Partial<GridConfig>,
+  ): Partial<GridConfig> | undefined => {
+    const albumGrid = config.albumGrids[albumId];
+    if (!albumGrid) return spGrid;
+    return { ...spGrid, ...albumGrid };
+  };
+
   if (!path || path.length === 0) {
     notFound();
   }
@@ -225,8 +236,8 @@ export default async function PathPage({ params, searchParams }: PathPageProps)
       <AlbumDetailView
         album={album}
         images={images}
-        layout={resolveLayout(spGrid)}
-        gridStyle={buildGridStyle(spGrid)}
+        layout={resolveLayout(mergeAlbumGrid(album.id, spGrid))}
+        gridStyle={buildGridStyle(mergeAlbumGrid(album.id, spGrid))}
         backLinkHref={`/${subpageSlug}`}
         backLinkLabel={`Back to ${subpageName}`}
         watermark={config.watermark}
@@ -341,8 +352,8 @@ export default async function PathPage({ params, searchParams }: PathPageProps)
         <AlbumDetailView
           album={album}
           images={images}
-          layout={resolveLayout(result.subpage.grid)}
-          gridStyle={buildGridStyle(result.subpage.grid)}
+          layout={resolveLayout(mergeAlbumGrid(album.id, result.subpage.grid))}
+          gridStyle={buildGridStyle(mergeAlbumGrid(album.id, result.subpage.grid))}
           subtitle={result.subpage.subtitle}
           backLinkHref="/"
           backLinkLabel="Back to Gallery"
@@ -357,7 +368,14 @@ export default async function PathPage({ params, searchParams }: PathPageProps)
     // Override albumThumbnailAssetId with the configured hero image (if any)
     const albumsWithHero = albums.map((album) => {
       const heroId = config.albumHeroImages[album.id];
-      return heroId ? { ...album, albumThumbnailAssetId: heroId } : album;
+      // EXPERIMENTAL: coverPosition rides along so the card crop can honour
+      // the configured focal point.
+      const coverPosition = config.albumCoverPositions[album.id];
+      return {
+        ...album,
+        ...(heroId ? { albumThumbnailAssetId: heroId } : {}),
+        ...(coverPosition ? { coverPosition } : {}),
+      };
     });
 
     // Batch-fetch ThumbHash for album cover placeholders
@@ -412,8 +430,8 @@ export default async function PathPage({ params, searchParams }: PathPageProps)
     <AlbumDetailView
       album={album}
       images={images}
-      layout={resolveLayout()}
-      gridStyle={buildGridStyle()}
+      layout={resolveLayout(mergeAlbumGrid(album.id))}
+      gridStyle={buildGridStyle(mergeAlbumGrid(album.id))}
       backLinkHref="/"
       backLinkLabel="Back to Gallery"
       watermark={config.watermark}