refactor: move git panel to tabbed bottom panel alongside terminal

Replace the narrow 256px sidebar git section with a shared tabbed
bottom panel that hosts both Terminal and Git as peer tabs. This gives
git operations full horizontal width for file paths, diff previews,
branch lists, and commit history.

- Create BottomPanel with shared resize handle, tab bar, and content
  switching (terminal stays mounted via CSS visibility when inactive)
- Simplify TerminalPanel by extracting resize/close to BottomPanel
- Add InlineGitPill toggle in composer footer next to Terminal pill
- Wire tab switching: clicking active tab closes panel, clicking
  inactive tab switches to it
- Remove GitPanel from ActivityPanel sidebar
- Update ARCHITECTURE.md to describe tabbed bottom panel layout

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/components/AppShell.tsx

@@ -4,11 +4,11 @@ interface AppShellProps {
   sidebar: ReactNode;
   content: ReactNode;
   detailPanel?: ReactNode;
-  terminalPanel?: ReactNode;
+  bottomPanel?: ReactNode;
   overlay?: ReactNode;
 }
 
-export function AppShell({ sidebar, content, detailPanel, terminalPanel, overlay }: AppShellProps) {
+export function AppShell({ sidebar, content, detailPanel, bottomPanel, overlay }: AppShellProps) {
   return (
     <div className="relative flex h-screen bg-[var(--color-surface-0)] text-[var(--color-text-primary)]">
       {/* Full-width drag region matching the title bar overlay height */}
@@ -19,7 +19,7 @@ export function AppShell({ sidebar, content, detailPanel, terminalPanel, overlay
         {sidebar}
       </aside>
 
-      {/* Main content + terminal */}
+      {/* Main content + bottom panel */}
       <main className="relative flex min-w-0 flex-1 flex-col">
         {/* Ambient glow behind active content area */}
         <div
@@ -27,7 +27,7 @@ export function AppShell({ sidebar, content, detailPanel, terminalPanel, overlay
           style={{ background: 'var(--gradient-glow)' }}
         />
         <div className="relative min-h-0 flex-1">{content}</div>
-        {terminalPanel}
+        {bottomPanel}
       </main>
 
       {/* Detail panel */}