Lazy-load Import stylesheet

[elkimek]

What changed

• Remove css/import.css from the eager app shell and load it on the first import/review interaction.
• Keep shared import buttons, header status, and empty-Labs drop-zone styles in the eager shell.
• Preserve cascade order with an HTML anchor, single-flight concurrent loads, and remove/cache-bust failed loads so a later interaction can retry.
• Route direct cycle-import previews through an explicit runtime dependency and keep the stylesheet in the service-worker app shell for offline first use.
• Add real browser coverage for cold startup, file-entry UI, concurrent loading, failed-load retry, direct cycle import, and Firefox offline behavior.

Impact

The exact fresh mobile returning-user load moves from 440 requests / 1,854,314 transfer bytes / 5,635,229 decoded bytes to 439 requests / 1,850,068 transfer bytes / 5,612,189 decoded bytes. That saves one startup request, 4,246 transfer bytes, and 23,040 decoded bytes while preserving the import experience.

Validation

• TypeScript and check-JS typechecks
• Architecture map: 465 modules, zero cycles
• Pre-release audit: 339 passed
• Vitest: 487 passed
• Chromium Playwright: 401 passed
• Responsive browser matrix: 472 passed
• Combined function coverage ratchet: 67.10% (minimum 67.10%)
• Firefox smoke/offline suite: 3 passed
• Cold-load budget reproduced exactly at 439 requests / 1,850,068 transfer bytes / 5,612,189 decoded bytes