generate index.html's absolute URLs from PUBLIC_URL

index.html hardcoded the deployment's domain in eight places, so every instance
had to carry its own edit of the file — the only thing separating the rezio
branch from master.

The tags genuinely need absolute URLs. OpenGraph is fetched standalone by
crawlers, and Bun's HTML bundler treats a root-relative href as an asset to
resolve on disk, failing the build with "Could not resolve: /favicon.ico" —
external URLs are the only form it passes through untouched.

Bun's HTML import offers no substitution hook, so scripts/gen-index.ts swaps
__PUBLIC_URL__ for the value in .env and writes index.gen.html, which the server
imports. index.html is the tracked template and is now identical on every
deployment; index.gen.html is gitignored. predev/prestart run the generator, and
it is idempotent so --watch does not loop.

Substituting also fixes the manifest: an absolute URL puts its fetch in CORS
mode, which failed whenever the hardcoded domain was not the serving origin.

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

package.json

@@ -9,7 +9,10 @@
   ],
   "scripts": {
     "preinstall": "node -e \"var v = +process.versions.node.split('.')[0]; if (v < 22 || v > 22) { console.error('Node 22 required (got ' + process.versions.node + '). Run: nvm use 22'); process.exit(1); }\"",
+    "gen:index": "bun run ./scripts/gen-index.ts",
+    "predev": "bun run ./scripts/gen-index.ts",
     "dev": "bun --env-file=.env --watch src/server.tsx",
+    "prestart": "bun run ./scripts/gen-index.ts",
     "start": "NODE_ENV=production bun src/server.tsx",
     "prebuild": "bun run ./scripts/prebuild.ts",
     "build:web": "bun run ./scripts/build/web.ts",