refactor: simplify scrollbar styles across components

[pikann]

Summary

• Add a global, theme-aware custom scrollbar (thin, transparent track, rounded thumb, darkens on hover) in index.css, applied via the universal selector so every scrollable element in the app — tables, dropdowns, kanban board, modals, chat panes, sidebar — gets it automatically instead of relying on the browser default.
• Define the .no-scrollbar utility, which was referenced by the sidebar nav rail but never actually defined, so it was silently falling back to the plain scrollbar.
• Remove the duplicated per-component WebKit-only scrollbar classes that a few components had hand-rolled (chat thread, docs editor, task detail modal, add-task-link modal, add-workflow-node modal), now that the global rule covers them. scrollbar-gutter:stable is kept where it was preventing layout shift.

Test plan

• bun run lint
• tsc -b
• Visual check in browser across a table, a dropdown, and the chat thread (light + dark mode)

[pullfrog]

ℹ️ No critical issues — minor suggestions inline.

Reviewed changes — adds a global custom-scrollbar rule in index.css and removes duplicated per-component WebKit scrollbar classes from five components, plus defines the previously missing .no-scrollbar utility.

• Global scrollbar styles in apps/web/src/index.css apply scrollbar-width: thin and WebKit thumb/track styling via the universal selector.
• Removed local scrollbar classes in the chat thread, doc editor, task detail modal, add-workflow-node modal, and add-task-link modal now that the global rule covers them; scrollbar-gutter:stable is preserved.
• Defined .no-scrollbar to match its existing use in apps/web/src/components/ui/sidebar.tsx.

  ^{｜ Fix all ➔ ｜ Fix 👍s ➔ ｜ View workflow run ｜ Using Kimi K2 (free via Pullfrog for OSS) ｜ 𝕏}