Lazy-load Settings stylesheet

[elkimek]

Summary

• defer css/settings.css until the first Settings or Tweaks action and load it concurrently with the existing lazy Settings module
• preserve the stylesheet's cascade position, keep shell-owned button styling eager, and retain the stylesheet in the offline app shell
• retry stylesheet failures independently without re-evaluating a successfully loaded module
• add startup, retry, offline, and responsive regression coverage and ratchet the cold-load budgets

Why

The Settings JavaScript was already lazy-loaded, but its 48 KB stylesheet still loaded for every returning-user startup even when Settings was never opened.

Impact

Identical same-machine cold-load runs improved from:

• 449 to 448 requests
• 1,908,524 to 1,900,879 transferred bytes
• 5,890,546 to 5,843,728 decoded bytes

That saves one startup request, 7,645 transferred bytes, and 46,818 decoded bytes.

Validation

• npm run test:playwright — 392 passed; responsive/theme suite 472 passed
• npm run test:firefox — 3 passed, including offline first-use Settings
• Settings-focused Playwright coverage — 15 passed
• import benchmark parallel stress rerun — 34 passed
• npm run typecheck
• npm run typecheck:checkjs
• npm run architecture:check
• npm run quality
• repeated cold-load measurement produced the same 448 / 1,900,879 / 5,843,728 result
• git diff --check