Fixes browser cache handling for generated msdf files

[uguraslan]

The dev server's configureServer middleware sends Cache-Control: public, max-age=31536000, immutable for generated MSDF font files. immutable tells the browser the response will never change, so once the files are cached, a normal reload will not request it again even after any change occurs in .ttf/.config.json files and the plugin regenerates files again server-side. Only a hard reload (empty cache) gets the updated files.

This fix replaces immutable with no-cache plus ETag (content hash). The browser still caches the response but always revalidates: unchanged fonts get served from its own cache via a 304, changed fonts trigger a full fetch.

I verified manually in blits-example-app: changed a font's .config.json, confirmed a normal reload now gets the regenerated atlas automatically, and confirmed a reload with no font changes still returns HTTP 304 instead of re-download. Etag implementation should work even very old browser versions.

[github-actions]

Test Results: ✅ PASSED

Run at: 2026-07-05T20:47:07.427Z

Summary:
passed: 1328 failed: 0 of 1328 tests

[suresh-gangumalla]

LGTM