quadraui: lift source control panel (inline commit input + split sections)

[JDonaghy]

Deferred from Phase C (#275). One of three remaining sidebar surfaces still hand-rolled per backend.

Goal

Migrate the Source Control panel to paint through quadraui primitives.

Why deferred — most complex of the three

• Inline commit-message editor at row 1 (multi-line input, mode-switching c, BackSpace, Return, Esc) — needs an InlineInput primitive or the existing Form row-editor pattern extended.
• Three sections (Worktrees / Staged / Unstaged) with per-section expand/collapse and bulk operations (stage-all on header).
• Split panes for diff view (GTK bundled nerd-font subset is missing glyphs referenced by code (silent Pango substitution) #197).
• Picker dropdown for branch switch (anchored to right side).

Likely needs the same MultiSectionView from #281 (debug sidebar) plus a new InlineInput primitive.

Estimated effort

~24 hrs

References

• TUI: src/tui_main/panels.rs — render_source_control
• GTK: src/gtk/draw.rs::draw_source_control_panel
• SourceControlData in src/render.rs
• Engine state: Engine::sc_* fields