feat(metassr-server): implement granular CSS rebuild in dev mode

[abhicodes0324]

Summary

RebuildType::Style previously logged a warning and did nothing - any CSS change in dev mode was silently ignored and the browser never updated styles. This PR implements it.

Changes

rebuilder.rs

• Added rebuild_styles() - re-runs the client-side bundle via ClientBuilder so Rspack picks up the changed CSS. Only the client bundle runs - no server-side rebuild is triggered
• RebuildType::Style arm now calls rebuild_styles() and broadcasts RebuildType::Style to connected WebSocket clients
• The browser receives the style message and reloadStylesheets() fires a CSS-only reload - no full page refresh
• Fixed typo in Page doc comment (providied → provided)
• Converted // comments on enum variants to /// doc comments

How to verify

1. Run metassr dev inside tests/web-app/
2. Open DevTools → Network tab → filter by CSS
3. Edit any file in src/styles/
4. Before: nothing happens in the browser
5. After: a style.css?t=<timestamp> request fires without a full page reload

Notes

• PR fix(live-reload): fix typo causing stylesheet cache-busting to silently fail in dev mode #105 (stylesheet cache-bust fix) is already merged, so reloadStylesheets() on the client works correctly
• rebuild_styles() currently passes "" as the root path to ClientBuilder (same as rebuild_page()) - this will be fixed in a follow-up PR

Checklist

• Addresses Style Rebuild TODO item in TODO.md
• 6 unit tests added for map_path_to_type and broadcast path
• cargo clippy clean
• cargo fmt applied