feat(chrome): put Settings beside update check and center unified-diff lines

[Warexpor]

What changed

Port of hardbeat920/monocode#72 onto this fork’s current chrome:

1. Sidebar footer — Settings sits on the same row as a compact size-8 update-check icon in SidebarUpdateFooter. Label + app version live on title / aria-label. The separate Settings RailAction blocks were removed from ProjectRail.tsx and Sidebar.tsx.
2. Unified diff — Hunk headers, gutter numbers, and code lines in UnifiedDiffView.tsx use flex items-center + leading-none instead of explicit lineHeight.

Why

Match upstream hardbeat920#72 without redesigning MonoCode chrome (still RailAction, UpdateRailCard, and accent states). Checking / available / download affordances stay on the update button via title.

Checklist

• I ran npm run check:web (vitest + tsc; 117 files / 1208 tests)
• npx vite build succeeded
• This PR is small and focused
• I did not mix unrelated changes
• Browser-smoked React chrome in Vite (not the Tauri shell)

Verification

Browser (Vite, no Tauri) — temporary harness mounted SidebarUpdateFooter + UnifiedDiffView (not committed):

• Settings RailAction and update icon sit on one row (Settings ~203×32, update 32×32, same top).
• Clicking Settings still fires onOpenSettings.
• After the browser updater catch path: title / aria-label = Check for updates · v0.0.0 (idle; getVersion falls back, check() is not configured).
• Unified diff code/gutter rows use flex + align-items: center and leading-none.

The full index.html App boot still depends on Tauri (loadBootWorkspace / invoke); that shell was not exercised.

Needs Tauri (not claimed here): live check() probe, checking/available/download icon states against a real endpoint, install/relaunch, native dialogs, Windows/DWM.

Refs: hardbeat920#72