ResourceControls: per-section panels use native scrollbars instead of Mantine ScrollArea

[cliffhall]

Background

In #1462 / PR #1464, the Resources controls accordion was reworked so each open section's panel scrolls within its own share of the height (pinned headers, flex-distributed). The per-section scroll uses native overflow-y: auto on .mantine-Accordion-panel (see App.css .disclosure-sections).

Problem

The rest of the Resources UI (and the Tools/Prompts/Network controls) uses Mantine ScrollArea / ScrollArea.Autosize, which renders a styled, thin, theme-consistent scrollbar. The per-section accordion panels now use the browser-native scrollbar, which looks slightly different (chunkier on Windows/Linux; overlay-on-scroll on macOS) and inconsistent with the rest of the app.

Proposed change

Replace the native overflow-y: auto on the section panels with a Mantine ScrollArea that fills the flex-distributed panel height (e.g. ScrollArea with flex: 1; min-height: 0 inside each open Accordion.Panel, or styled scrollbar CSS), so the per-section scrollbars match the rest of the app.

Care needed: the panels are flex-sized and the accordion disables its Collapse height animation (transitionDuration={0}) to avoid fighting the flex layout — a nested ScrollArea must respect that flex height (fill, not measure content) so the section-sharing behavior from #1462 is preserved.

Acceptance criteria

• Per-section scrollbars in the Resources accordion match the Mantine ScrollArea style used elsewhere.
• The ResourceControls: sections scroll prematurely (equal height-division) + accordion chevrons point up/down instead of right/down #1462 behavior is preserved: pinned headers, per-section scroll, space shared by item count, no scroll until the panel is full, card grows-with-content capped at screen height.
• npm run validate + npm run test:storybook pass.

Notes

Low priority / fit-and-finish. Split out from #1462 so the core layout fix can land first.