fix(scrollbar): Fix weird scrollbar issue on onboard page

[plind-junior]

Summary

On the Bounties page (and intermittently elsewhere), the browser's default light/system scrollbar would appear when scrolling — most reliably reproducible by applying a filter and then scrolling up and down. The dark theme scrollbar from scrollbarSx was only being applied to specific containers, so any other scroll context (the body itself, or a transient overflow context after a filter resize) fell back to the OS default.

This PR adds a global ::-webkit-scrollbar + Firefox scrollbar-color baseline in src/index.css so every scrollable element on the page picks up the dark theme automatically.

Cause

scrollbarSx (src/theme.ts:82) is an MUI sx snippet that styles the scrollbar of the element it's attached to. Components have to opt in. Today only a handful of containers do — <main> in AppLayout.tsx:154, DataTable's TableContainer, the global search results dropdown, etc.

When filtering on /bounties causes the result count (and therefore page height) to shrink, the scroll context can briefly move from <main> to the body for a frame, and the body's scrollbar isn't styled — producing the visible "default scrollbar appearing" flash.

Related prior work

This is complementary to past per-container fixes — none of which targeted the global stylesheet:

• fix: unify scrollbar styles across code viewer and app #249 — extracted scrollbarSx and applied it to ~12 components
• fix: inconsistent scrollbar usage across 10+ files and components #463 — added scrollbarSx to AppLayout's <main> and 16 other containers
• fix(scrollbar): unify main content and sidebar scrollbar styling #228 (closed) — attempted unified scroll container approach
• refactor: replace inline scrollbar styles with shared scrollbarSx #286 (closed) — refactor follow-up to fix: unify scrollbar styles across code viewer and app #249

Test plan

• Open /bounties, apply a filter (e.g. "Available"), scroll up and down — no white/native scrollbar appears.
• Repeat on Watchlist, Repositories, Top Miners — scrollbar styling is consistent.
• Sanity: existing scrollbars on <main>, DataTable, and the global search results dropdown are visually unchanged.
• Firefox: scrollbar shows the thin dark variant via scrollbar-color.

Out of scope

Not changing any per-container scrollbarSx usage — those keep working and are kept for explicit intent on specific scroll regions.

Before

scroll.issue.mp4