site: owner-only dashboard link in header, mobile menu and homepage (g then d from any page); /start skips the site chrome JS

- Header/footer moved to #lib/components/SiteChrome.svelte, returned by a universal
  +layout.ts load only for non-immersive pages, so /start and the book reader never
  download icon libraries, dropdowns or the bits-ui Sheet. /start: 36 -> 18 JS files,
  133 -> 78 KB gzip.
- Header logo is a hashed asset (inlined), so it is cached with the build.
- inlineStyleThreshold 4096: tiny page/component CSS is inlined into the HTML.
- Umami is not loaded on /start.

src/lib/chrome.test.ts

@@ -0,0 +1,13 @@
+import { describe, expect, it } from 'vitest';
+import { isImmersive } from './chrome';
+
+describe('immersive pages', () => {
+	it('skips the site chrome on /start and the book reader only', () => {
+		expect(isImmersive('/start')).toBe(true);
+		expect(isImmersive('/start/')).toBe(true);
+		expect(isImmersive('/trackers/books/12/read')).toBe(true);
+		expect(isImmersive('/')).toBe(false);
+		expect(isImmersive('/starter')).toBe(false);
+		expect(isImmersive('/trackers/books/12')).toBe(false);
+	});
+});