Add optional PWA support to Dist

[yozhgoor]

Summary

• Add a new optional pwa feature and Dist::pwa(Pwa) builder to enable PWA artifact generation during dist builds.
• Introduce Pwa/PwaIcon configuration in src/pwa.rs, with generation of manifest.json and sw.js only when those files are missing (custom assets remain authoritative).
• Generate service worker precache entries from the final dist contents (sorted + deduped), excluding sw.js, *.map, and *.txt.
• Re-export PWA types from lib.rs behind the pwa feature and wire PWA application into the Dist::build pipeline.
• Extend dev server content types to better support PWA assets (json, png, svg).

Behavior

• Existing workflows are unchanged unless the pwa feature is enabled and .pwa(...) is used.
• In PWA mode, generated files are fallback defaults only; user-provided assets always take precedence.