fix(ssg): fix white header on dark theme and resolve critters CSP block (#66)

## Summary
- **Critters preload**: Set `preload: false` in `ssgPlugin` to eliminate
`onload="this.media='all'"` event handlers on external stylesheets,
which violated Content Security Policy (`script-src 'self'` /
`require-trusted-types-for 'script'`) and left the compiled stylesheet
permanently blocked in `media="print"`.
- **Dark theme selector retention**: Added `allowRules: [/\.dark/]` to
Critters options so dark theme selectors and palette variables are
preserved during critical CSS inlining.
- **Immediate theme variables**: Initialized `--rb-background`,
`--rb-base`, and `--rb-text` in `THEME_INIT_STYLES` for `:root` and
`.dark` alongside background color to ensure components like `PageHead`
instantly reflect the active theme.

## Verification
- `pnpm typecheck`: Passed (0 errors)
- `pnpm lint`: Passed (0 warnings)
- `pnpm test`: Passed (all suites passed)
- `pnpm build`: Passed (clean SSG generation)
- Local Playwright browser test with dark color scheme verified
`headerBg` is `rgb(20, 20, 20)` and zero CSP inline handler violations.

packages/ui/src/lib/theme/constants.ts

@@ -10,7 +10,7 @@ export const THEME_INIT_SCRIPT = `(function(){try{var k='ermnvldmr/ui/lib/theme'
  * These align with the main palette in styles.css.
  */
 export const THEME_INIT_STYLES = `
-  :root { background-color: #fafafa; color: #262626; }
-  .dark { background-color: #141414; color: #f6f6f6; }
+  :root { --rb-background: #fafafa; --rb-base: #f6f6f6; --rb-text: #262626; background-color: #fafafa; color: #262626; }
+  .dark { --rb-background: #141414; --rb-base: #262626; --rb-text: #f6f6f6; background-color: #141414; color: #f6f6f6; }
   body { margin: 0; transition: background-color 0.2s ease; }
 `;