TUI ext panel: scrollbar not drawn even when content overflows viewport

[JDonaghy]

Problem

In TUI, when an extension-provided sidebar panel has more content than fits in a vertically-small window, the scrollbar column doesn't render. The click handler at `src/tui_main/mouse.rs` already supports `ext_panel:sb` drags (added in 03a3339 sidebar-scrollbar pilot), but the corresponding visual scrollbar isn't painted by the renderer — so users can't initiate the drag.

Repro

1. Install an extension that provides a sidebar panel (e.g. one of the bundled extensions, if available).
2. Open it; resize the terminal vertically so panel rows > visible content rows.
3. Expected: scrollbar column visible at the right edge of the panel.
4. Actual: no scrollbar; content scrolls only via keyboard nav.

Suspected Cause

`src/tui_main/panels.rs` ext-panel render path doesn't have a `render_*_scrollbar` call equivalent to the explorer's `render_explorer_scrollbar` (panels.rs:645). The `engine.ext_panel_scroll_top` field exists and is populated, but no UI renders the thumb.

Files

• `src/tui_main/panels.rs::render_ext_panel` — needs scrollbar drawing block.
• `src/render.rs` — `ExtSidebarData` already exposes the needed `flat_len`.

Priority

Minor — keyboard navigation works; only mouse-only users hit it. Pre-existing, not introduced by 03a3339.