feat(ui): refine search, startup, and scrollbar behavior

[GCWing]

Summary

• Centralize native scrollbar presentation in @openbitfun/ui, remove product-local paint overrides, and refine shared SearchField styling.
• Make FlowChat search highlight concrete text occurrences and avoid moving the viewport when the current hit is already readable; keep Assistant sessions on their fixed Claw mode and align expanded toolbar geometry.
• Generate pre-React startup copy from the canonical locale contract, keep startup controls synchronized with the resolved locale, and refine splash/welcome motion.

Type and Areas

Type: UI/UX, feature, bug fix, test, docs

Areas: design system, Web UI / FlowChat, desktop startup, i18n, docs

Motivation / Impact

Search now emphasizes the exact matching text and current readable line without unnecessary transcript jumps. Native scrollbars follow one accessible design-system policy instead of diverging by panel. Startup copy is generated from the locale source of truth, and the welcome/splash presentation settles into a calmer final state.

Verification

• pnpm run i18n:generate — generated outputs are in sync
• pnpm run i18n:contract:test — 37/37 passed
• pnpm run i18n:audit — passed with 0 warnings
• pnpm run design-system:check — passed, including package/lab builds and 252 UI tests
• pnpm run check:web — passed, including type-check, Appearance, theme color, and visual-governance contracts
• Focused Web UI tests for startup, scrollbar ownership, composer policy, FlowChat search/navigation, and toolbar geometry — 8 files / 122 tests passed
• pnpm run check:repo-hygiene — passed
• cargo check -p openbitfun-desktop — passed
• cargo test -p openbitfun-desktop — test binary compiled, but Windows failed before executing tests with STATUS_ENTRYPOINT_NOT_FOUND (0xc0000139); no test assertion ran. GitHub CI is required before merge.
• Browser interaction / visual QA was not run, per repository guidance.

Remote scenarios: local UI contracts only. Remote workspace, Remote Control, Peer Device Mode, and Detached Dispatch were not exercised; this PR does not change their transport, persistence, or capability contracts.

Reviewer Notes

The branch is rebased on the latest main. The startup locale projection is generated inside src/web-ui/index.html; edit locale resources and rerun pnpm run i18n:generate rather than editing that block by hand.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.