Redesign ticket list to three-column layout with proportional widths

- Replace Sheet slide-over with inline peer detail column
- List column: 40% width (min 360px) when ticket selected, flex-1 otherwise
- Detail column: 60% width (min 480px), slides in from right (300ms)
- Mobile: list hidden when ticket selected, detail becomes full-width
- Subtle border-r divider on list column
- Taller ticket rows (py-4) with smooth hover transitions
- width transition on list column resize

web/src/components/app-shell.tsx

@@ -306,7 +306,7 @@ export function AppShell({ children }: { children: React.ReactNode }) {
         </aside>
 
         {/* Main */}
-        <main className="flex-1 overflow-y-auto">{children}</main>
+        <main className="flex-1 overflow-hidden">{children}</main>
 
         {/* Command Palette */}
         <CommandPalette open={commandOpen} onOpenChange={setCommandOpen} />