refactor(tui): migrate Extension Panel to quadraui TreeView (#476)

[JDonaghy]

Summary

• New render::ext_panel_to_tree_view() adapts ExtPanelData into quadraui::TreeView (section headers + indented items + chevrons + combined badges/action labels).
• TUI render_ext_panel() rewired: chrome via quadraui::tui::draw_settings_chrome, body via Backend::draw_tree. Manual scrollbar + scroll-surface registration kept (TreeView doesn't paint scrollbars yet); help-popup overlay unchanged.
• build_ext_panel_data() now resolves engine.ext_panel_tree_expanded overrides into the cloned item.expanded so the builder stays engine-free, matching the ext_sidebar_to_tree_view() pattern.

Net: panels.rs −252 / +101, render.rs +154 (builder + 2 unit tests).

Known visual difference

Separator rows (item.is_separator) render as a single ─ glyph instead of the full-width rule the legacy renderer drew — the TreeView primitive has no Decoration::Separator. No shipping plugin uses separators today; if we want the full rule back, file a quadraui gap for the primitive.

Pre-existing bugs surfaced during smoke testing (not introduced by this PR)

mouse.rs is unchanged by this PR. Three behaviors flagged during smoke test that are byte-for-byte identical to develop:

1. / activates search input but typing doesn't filter — engine fires panel_input to the plugin; filtering is the plugin's job.
2. Click on a section header doesn't toggle expand (Enter does) — click handler calls the same handle_ext_panel_key(\"Return\", ...) Enter does. Likely an Alacritty Down/Up quirk similar to TUI: right-click on explorer does nothing (engine state opens but menu doesn't render or dispatch) #451.
3. Scroll wheel doesn't work though scrollbar drag does — registration + dispatch identical to develop.

Filed as separate follow-ups.

Test plan

• cargo build --no-default-features
• cargo test --no-default-features (43 binaries, 0 failures, 2 new render tests covering tree shape + focus state)
• cargo clippy --no-default-features -- -D warnings
• cargo fmt
• Smoke test against git_insights plugin: title renders, chrome shows search row when active, j/k navigates, Enter expands sections, scrollbar thumb tracks, help popup unaffected
• Need a plugin with separators to confirm the visual approximation looks acceptable (none in tree today)

Closes #476.

🤖 Generated with Claude Code

[JDonaghy]

Coordinator review — approve

Clean migration. −252 lines of manual cell-painting replaced by ext_panel_to_tree_view() builder + draw_tree call. Key things verified:

• All ExtPanelStyle variants mapped correctly (Header→Header, Dim→Muted, Accent→keyword-colored Normal, Normal→Normal)
• Badge/action/hint concatenation matches the legacy format
• Tree expansion state resolved in build_ext_panel_data() so the builder stays engine-free — correct pattern
• Separator approximation documented and acceptable (no shipping plugin uses separators)
• Scrollbar stays manual (TreeView primitive doesn't paint them yet) — consistent with the codebase
• Pre-existing bugs (TUI ext panel: typing in search input doesn't filter visible items #483, TUI ext panel: click on section header doesn't toggle expand (Enter works) #484, TUI ext panel: scroll wheel doesn't scroll the panel (scrollbar drag works) #485) correctly filed as separate issues, not conflated with this PR
• 2 new render tests verify tree shape + no-focus path

Ready to merge. Smoke test needed on Desktop A (GTK) to confirm no regression on that backend — this PR only changed TUI + render.rs, but the render.rs builder is shared.