fix(layout): align right sidebar to top in non-workspace views

[leynier]

Summary

Fixes a layout bug where the right sidebar opened below the full-width titlebar in non-workspace views (landing, settings, tasks), instead of anchoring at the top of the window like it does in workspace view.

Root cause

src/renderer/src/App.tsx built the shell as a flex-col with the full-width .titlebar rendered above the inner flex-row that hosts <RightSidebar />. Because the titlebar spanned the entire window width, it sat over the right sidebar's column too, so the right sidebar's own 42px header rendered at y≈42px instead of y=0.

When workspaceActive becomes true (a worktree is selected), the full-width titlebar is dropped and each column gets its own 42px top strip - that's why the bug only manifested in non-workspace views.

What changed

• Wrapped the left + center columns in a flex-col that owns the conditional titlebar, and lifted <RightSidebar /> to be a sibling of that wrapper inside the existing flex-row. The titlebar now spans only the left+center area, so the right sidebar's own header anchors at the top of the window in landing/settings/tasks - matching the workspace-view alignment.
• Gated the titlebar's right-sidebar toggle on !rightSidebarOpen so it no longer double-renders alongside the close button inside the right sidebar's own header when the sidebar is open. (Same pattern already used in workspace view at the floating-toggle branch.)

Workspace view layout is unchanged (zero visual diff).

Resolve #1354

Screenshots - verification

Landing, right sidebar closed - full-width titlebar as before:

Landing, right sidebar open - right sidebar anchors at the top, titlebar shrinks to the left+center area, no duplicate toggle icon:

Workspace, right sidebar closed - unchanged from before:

Workspace, right sidebar open - unchanged from before:

Test plan

• pnpm run typecheck - passes
• pnpm run lint (oxlint) - 0 warnings, 0 errors
• pnpm test - 3675/3675 tests pass (under Node 24 + pnpm rebuild better-sqlite3)
• Manual verification on macOS:
  • Landing screen, toggle right sidebar open: header icons at y=0, aligned with traffic-light row
  • Landing screen, toggle right sidebar closed: titlebar re-expands to full width, no duplicate toggle
  • Workspace view (worktree + terminal): right sidebar still anchored at the top, no regression
  • Workspace view, file explorer panel open in right sidebar: working
  • Window drag region preserved