feat: theming system, documentation, and photo grid regression tests

- Add configurable theme system with 4 presets (studio, minimal, editorial, classic)
- Theme controls fonts, colors, accent, photo frames, grain, and header dot
- Resolve themes from gallery.yaml: string shorthand or full object override
- Inject theme as CSS custom properties on <html> element
- Dynamic Google Fonts loading based on theme config
- OG image route reads accent/background from theme
- Refactor Studio overrides into conditional CSS via data-attributes

Documentation:
- Add docs/gallery-config.md with comprehensive configuration reference
- Add docs/theming.md with preset reference and custom theme guide
- Slim README to link to detailed docs

Fix CSS regression:
- Fix passepartout frame overflow:visible breaking next/image fill rendering
- Remove duplicate .photo-grid__item block that stripped overflow:hidden

Testing:
- Add e2e/photo-grid.spec.ts with 3 regression tests:
  - Grid items render with real dimensions
  - overflow:hidden CSS guard (catches the exact bug fixed)
  - Images load with real pixel data (not blurhash)

Tooling:
- Add scripts/screenshots.ts for automated theme screenshot generation
- Handles dev server lifecycle, theme cycling, FadeIn + lazy image handling

docs/gallery-config.md

@@ -0,0 +1,113 @@
+# Gallery Configuration
+
+All gallery structure is defined in `content/gallery.yaml`. Copy the example to get started:
+
+```bash
+cp content/gallery.yaml.example content/gallery.yaml
+```
+
+## Hero Images
+
+Single image or crossfade carousel on the homepage:
+
+```yaml
+# Single hero image
+hero: 00000000-0000-0000-0000-000000000000
+
+# Carousel (crossfade between multiple images)
+hero:
+  - 00000000-0000-0000-0000-000000000000
+  - 11111111-1111-1111-1111-111111111111
+  - 22222222-2222-2222-2222-222222222222
+```
+
+## Standalone Albums
+
+Albums shown directly on the homepage as cards:
+
+```yaml
+albums:
+  - 11111111-1111-1111-1111-111111111111
+  - 22222222-2222-2222-2222-222222222222
+```
+
+## Subpages
+
+Group multiple albums into named collections. URLs are auto-generated from the name:
+
+```yaml
+subpages:
+  - name: Japan                    # → /japan
+    albums:
+      - 33333333-3333-3333-3333-333333333333  # Tokyo
+      - 44444444-4444-4444-4444-444444444444  # Kyoto
+
+  - name: Wedding – Smith         # → /wedding-smith
+    password: clientpass123        # password-protected
+    albums:
+      - 55555555-5555-5555-5555-555555555555
+```
+
+## Grid Layout
+
+Configure the photo grid globally or per-subpage:
+
+```yaml
+# Global defaults
+grid:
+  columns: 3            # number of columns (default: 3)
+  gap: 12               # gap in pixels (default: 12)
+  aspectRatio: "1"       # "1", "3/2", "auto", etc. (default: "1")
+  layout: masonry        # "masonry" or "uniform" (default: "masonry")
+
+subpages:
+  - name: Japan
+    grid:                # per-subpage override
+      columns: 4
+      layout: uniform
+      aspectRatio: "3/2"
+    albums:
+      - ...
+```
+
+## EXIF on Hover
+
+Show camera/lens/settings when hovering over photos (enabled by default):
+
+```yaml
+exifOnHover: false    # set to false to disable
+```
+
+## Footer
+
+Optional minimal footer with social links:
+
+```yaml
+footer:
+  name: John Doe
+  instagram: johndoe
+  email: hello@example.com
+  website: https://example.com
+```
+
+## About Page
+
+Create `content/about.md` with YAML frontmatter:
+
+```markdown
+---
+portrait: asset-uuid-for-your-portrait
+name: Your Name
+location: City, Country
+gear:
+  - Camera Body
+  - Favorite Lens
+---
+
+Your bio text here. Supports full Markdown.
+```
+
+## Finding UUIDs
+
+- **Album UUIDs**: In Immich, go to Albums → click an album → the UUID is in the URL bar
+- **Asset UUIDs**: Click any photo → the UUID is in the URL bar