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

app/tokens.css

@@ -1,6 +1,7 @@
-@import url('https://fonts.googleapis.com/css2?family=DM+Sans:wght@300;400;500;600&family=Playfair+Display:wght@400;500;600&family=EB+Garamond:ital,wght@0,400;1,400&display=swap');
+/* Fonts are loaded dynamically via layout.tsx based on theme config */
 
-/* ── Design Tokens (Studio Dark — Default) ───────── */
+/* ── Design Tokens (Fallback Defaults) ───────────── */
+/* These are overridden at runtime by CSS vars injected from gallery.yaml theme config. */
 :root {
   --bg-primary: #1a1a1a;
   --bg-secondary: #141414;
@@ -83,4 +84,4 @@ a {
 img {
   display: block;
   max-width: 100%;
-}
+}