Theme pairs with dark mode toggle, gear active state, PALETTE_DEFAULT_DARK (#127)

- Collapse the 10 theme cards into 5 pairs: light swatches top row, dark bottom row
- Dark mode toggle in settings and topbar (sun/moon inline SVG)
- Toggling dark mode switches to the other variant of the selected pair
- Persist pair id in localStorage palette-theme, dark flag in palette-dark
- Head script resolves base+dark to variant; URL ?theme= accepts both ids and wins
- Settings gear highlights like nav tabs on /settings
- PALETTE_DEFAULT_DARK env var sets server default dark state (default on)
- Server-side resolution helpers + tests

README.md

@@ -63,6 +63,7 @@ go build -o palette ./cmd/palette
 | `PALETTE_MAX_TEXT` | `5242880` | Max paste size in bytes (5 MB) |
 | `PALETTE_MAX_ITEM` | `26214400` | Max can item size in bytes (25 MB) |
 | `PALETTE_ADMIN_KEY` | generated | Admin key; if unset a 32-char hex key is generated and persisted to `<db-dir>/admin-key` (0600) |
+| `PALETTE_DEFAULT_DARK` | dark on | Default dark mode for new visitors. Set `false`, `0`, or `off` to default to light mode. Visitors who toggle dark mode keep their choice in their browser. |
 
 An `/admin` page exists for runtime settings, protected by a key set at
 install (`PALETTE_ADMIN_KEY` env var) and resettable locally — see