feat(web): add quiet panel scroll affordances

[danyaberezun]

Summary

Hide scrollbar chrome at rest in ThinkRail’s auxiliary tool and terminal surfaces while keeping scrolling discoverable and directly manipulable. Scroll thumbs now reveal on local intent, and short directional curtains appear only where content is clipped.

Changes

• add dependency-light QuietScrollArea and QuietScrollFrame primitives with resize/content-aware edge detection
• apply quiet scrolling to Projects, Files, Specs, Changes, Review, and xterm wherever the terminal is placed
• preserve native and xterm scrollbar hit geometry while rendering a compact optical thumb on hover, focus, drag, touch, or active scroll
• retain persistent hairlines for bundled high-contrast themes, visible system scrollbars in forced-colours mode, and transition-free behavior under reduced motion
• derive xterm edge state through its public buffer API while leaving Monaco and the existing tab-strip overflow treatment unchanged
• update the web module boundaries/specs and add focused unit and browser coverage
• make recalled-prompt delivery coverage assert the exact outgoing session.prompt frame instead of a legitimately virtualized transcript row

Testing

• bun run check:deps — passed
• bun run check:seams — passed
• bun run lint — passed with 0 errors (6 pre-existing unused-suppression warnings)
• bun run typecheck — 11/11 tasks passed
• bun run test — 11/11 tasks passed
• bun run e2e — full 300-test no-agent suite passed on this head across 8/8 shards
• post-refresh stress rerun: three unrelated contention/race failures; bun run e2e -- --last-failed passed all 3 serially

Checklist

• Fast gates pass: bun run lint, bun run typecheck, bun run test
• E2E suite passes for app-affecting changes
• Relevant SPEC.md files updated for the boundary and behavior change
• I have read the Contributing guide and agree to the Code of Conduct

Screenshots

Files panel — idle, mid-scroll

Before	After

Terminal — idle, scrolled away from latest output

Before	After

[jetbrains-air]

Approved — ready to merge.

[jetbrains-air]

Approved — ready to merge.