Landing performance: flag font subset, responsive mascots, AVIF cache, HTML minify

- Flag font: ship a committed fontTools subset (43 KB, -45%) of
  TwemojiCountryFlags.woff2 with only the 15 flags the site uses, under a
  content-hashed name with a preload on every flag-bearing page. The build
  validates the subset against a manifest and fails when a new flag
  appears without regenerating (npm run subset-flags). JS subsetters
  (subset-font/harfbuzzjs) were rejected: their wasm builds silently drop
  the COLR/CPAL color tables, rendering all flags invisible.
- Responsive mascots: legal/join/404 pages served 434-500px sources for
  175-180px slots; the build now emits 180w/360w variants (AVIF 1x: ~7 KB
  vs ~22 KB) and the pages use srcset. Small logo spots (14-42px) now use
  the existing 64/128px variants instead of the 256px file.
- injectAvifPictures copies the img sizes attribute onto the AVIF
  <source>; without it Chrome mis-selects w-descriptor candidates and can
  drop the image entirely.
- AVIF conversion is cached by content hash (.avif-cache.json); the old
  mtime check never hit because copyDirSync refreshes mtimes, so every
  verify re-encoded all 26 files.
- HTML output is minified (comments + indentation, <pre> preserved,
  newlines kept for inline-script safety): index.html 121 KB -> 93 KB.
- unicode-range trimmed to U+1F1E6-1F1FF; tag-sequence flags fall through
  to the system emoji font.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -9,6 +9,7 @@
     "indexnow": "node website/submit-indexnow.cjs",
     "lint": "eslint .",
     "lint:fix": "eslint . --fix",
+    "subset-flags": "node website/tools/subset-flag-font.mjs",
     "test": "npm run verify",
     "test:unit": "vitest run",
     "verify": "node scripts/verify-release.mjs"