fix(web): make annotation post-it readable

[lastobelus]

The persistent thread annotation card used a mostly transparent warning background, so message content remained visible through the card and made longer annotations unreadable.

This reuses the app's shared blurred overlay surface beneath the warning tint, keeps the card flat so it does not compete with the elevated composer, and lets the action row wrap safely at narrower widths. A focused rendering test locks in the readable overlay layering and absence of annotation-card elevation.

Visual QA:

• Before: underlying transcript text visibly collided with annotation content.
• After: approved in the isolated web preview with the original long annotation content; transcript text no longer bleeds through and only the composer retains elevation.

Validation:

• vp test run apps/web/src/components/thread-annotation/ThreadAnnotationPostIt.test.tsx apps/web/src/components/thread-annotation/ThreadAnnotation.test.ts
• vp lint apps/web/src/components/thread-annotation/ThreadAnnotation.tsx apps/web/src/components/thread-annotation/ThreadAnnotationPostIt.test.tsx
• vp run -F @t3tools/web typecheck
• Run Quick CI

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