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/theming.md

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+# Theming
+
+Control the visual identity of your gallery from `content/gallery.yaml`. Choose a built-in preset or customize individual properties — no CSS editing required.
+
+## Quick Start
+
+Add a single line to your `gallery.yaml`:
+
+```yaml
+theme: minimal
+```
+
+That's it. Your entire gallery switches to the minimal theme.
+
+## Built-in Presets
+
+| Preset | Look | Accent | Fonts | Corners | Frame | Grain |
+|--------|------|--------|-------|---------|-------|-------|
+| **studio** | Leica-inspired, editorial | 🔴 `#e60012` | Playfair Display + DM Sans | Sharp | Passepartout | ✓ |
+| **minimal** | Clean, no distractions | ⚪ `#666666` | Inter | Sharp | None | ✗ |
+| **editorial** | Magazine feel | ⚫ `#1a1a1a` | Libre Baskerville + Source Sans | Slight | Shadow | ✗ |
+| **classic** | Warm, inviting | 🟡 `#8b6914` | Cormorant Garamond + Lato | Rounded | None | ✗ |
+
+Default is `studio` if no theme is specified.
+
+## Custom Theme
+
+Start from any preset and override individual properties:
+
+```yaml
+theme:
+  preset: studio              # base preset to extend (default: studio)
+  accent: "#2563eb"           # brand/accent color (any hex)
+  fonts:
+    heading: "Inter"          # Google Fonts name for headings
+    body: "Inter"             # body text
+    caption: "JetBrains Mono" # EXIF captions
+  radius: 8                   # border-radius in px (0 = sharp corners)
+  photoFrame: none            # "none" | "passepartout" | "shadow"
+  grain: false                # film grain overlay on photos
+  headerDot: false            # accent-colored dot in the nav bar
+  heroStyle: split            # "split" | "fullbleed" | "minimal"
+```
+
+All properties are optional — omitted values fall back to the preset defaults.
+
+## Properties Reference
+
+### `accent`
+
+The brand color used for hover effects, navigation highlights, the header dot, and OG social preview images.
+
+### `fonts`
+
+Google Fonts names. Fonts are loaded automatically — just use the name as it appears on [fonts.google.com](https://fonts.google.com). Three font slots:
+
+- **heading** — album titles, hero title, section labels
+- **body** — navigation, descriptions, UI text
+- **caption** — EXIF metadata, photo captions
+
+### `radius`
+
+Border radius in pixels applied to photo grid items and album cards:
+- `0` — sharp, editorial corners
+- `4-8` — subtle rounding
+- `12+` — soft, modern feel
+
+### `photoFrame`
+
+How photos are framed in the grid:
+
+- **`none`** — photos fill the grid cell directly
+- **`passepartout`** — museum-style mat border around each photo with print-style EXIF captions below
+- **`shadow`** — subtle drop shadow behind each photo
+
+### `grain`
+
+When `true`, a subtle film grain texture overlays each photo in the grid. Adds an analog, filmic character.
+
+### `headerDot`
+
+When `true`, shows a small accent-colored dot in the navigation bar (inspired by the Leica red dot).
+
+### `heroStyle`
+
+Controls the homepage hero layout:
+- **`split`** — title/nav on the left, hero image on the right (default)
+- **`fullbleed`** — hero image fills the entire viewport
+- **`minimal`** — text-only, no hero image
+
+## Examples
+
+### Minimal portfolio
+```yaml
+theme:
+  preset: minimal
+  accent: "#0066cc"
+  fonts:
+    heading: "Outfit"
+    body: "Outfit"
+```
+
+### Film photography blog
+```yaml
+theme:
+  preset: studio
+  accent: "#d4a017"
+  grain: true
+  photoFrame: passepartout
+```
+
+### Modern magazine
+```yaml
+theme:
+  preset: editorial
+  radius: 12
+  fonts:
+    heading: "Fraunces"
+    body: "Inter"
+    caption: "IBM Plex Mono"
+```