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

content/gallery.yaml.example

@@ -49,3 +49,24 @@ subpages:
 #   instagram: johndoe
 #   email: hello@example.com
 #   website: https://example.com
+
+# ── Theme ─────────────────────────────────────────────────────
+# Control the visual identity of your gallery. Use a preset name:
+#   theme: studio     # Leica-inspired, red accent, passepartout frames, film grain
+#   theme: minimal    # Clean, no distractions, grey accent
+#   theme: editorial  # Magazine feel, shadow frames
+#   theme: classic    # Warm, golden accent, rounded corners
+#
+# Or customize individual properties (defaults from the preset):
+# theme:
+#   preset: studio              # base preset to extend
+#   accent: "#2563eb"           # brand/accent color
+#   fonts:
+#     heading: "Inter"          # Google Fonts name for headings
+#     body: "Inter"             # body text
+#     caption: "JetBrains Mono" # EXIF captions
+#   radius: 8                   # border-radius in pixels (0 = sharp)
+#   photoFrame: none            # "none" | "passepartout" | "shadow"
+#   grain: false                # film grain overlay on photos
+#   headerDot: false            # accent dot in the nav bar
+#   heroStyle: split            # "split" | "fullbleed" | "minimal"