perf(web): drop install icons from the precache and minify theme-init.js

[mforce]

Refs #1322

The service worker precache drops from 1,677.86 KiB to 1,644.33 KiB (33.53 KiB less). PRECACHE_CEILING_KIB goes down from 1,680 to 1,659, which leaves 14.67 KiB of headroom.

What changed

Both changes are in web/vite.config.ts. web/scripts/verify-sw.mjs only gets the new ceiling and a history entry.

1. The five install icons (icon-*.png, apple-touch-icon.png) are added to globIgnores, which saves 30.73 KiB. The browser downloads them from the manifest when someone installs the app, and that happens online. They are still in dist and are still served. includeManifestIcons: false stays, because the plugin would otherwise add them back. Its old comment said Workbox precaches the icons anyway, so I rewrote it.
2. A new minifyThemeInit() plugin minifies dist/theme-init.js at build time, from 3,754 to 895 bytes. public/theme-init.js stays readable. The plugin uses writeBundle, which runs before the PWA plugin's closeBundle, so the precache revision is the hash of the minified file. The script is minified as a classic script, not a module, so its top-level vars are still globals, exactly as before. The URL (/theme-init.js) and the CSP do not change.

Verification

• The theme-init.js revision in dist/sw.js equals the MD5 of dist/theme-init.js.
• node scripts/verify-sw.mjs dist passes, reporting 1,644.33 KiB against the 1,659 KiB ceiling. I measured main (884b971) at 1,677.86 KiB on the same machine.
• node --test scripts/*.test.mjs passes 19/19. The opticalSize, themeInit and theme Vitest files pass 42/42, and npm run typecheck is clean.
• I also ran themeInit.test.ts once against the minified dist file. All 19 behavior tests passed. Only the slug-pattern parity test failed, which is expected because it compares source text.

Not in this PR

• Leaving the Inter latin-ext subset out of the precache. That changes web: give display text an optical size using Inter's opsz axis #835's offline typeface promise and needs a decision from the maintainer (see web: find precache savings and lower the service-worker ceiling #1322).
• Splitting the Help prose out of the bundle (see web: find precache savings and lower the service-worker ceiling #1322).

No screenshots: nothing a user sees changes.