fix(web-ui): polish nav search trigger and Monaco scrollbar

[bobleer]

Summary

• Restyle the nav panel search trigger as a compact button (soft element background, square radius, pointer cursor) instead of a capsule that mimics a text field; remove hover padding/wiggle animation for clearer affordance.
• Shorten the trigger placeholder copy to Search / 搜索 (full hint remains on the dialog input).
• Make Monaco scrollbar track transparent so the themed slider reads cleanly on the code editor.

Test plan

• Open desktop/web UI: nav search trigger looks correct in light/dark theme; hover/focus/active states OK.
• Open global search (Ctrl+K): dialog search field still shows full placeholder.
• Open a file in code editor: scrollbar track does not show a solid strip behind the thumb.