feat: add animated transitions and micro-interactions

Add entrance animations to overlays, modals, chat messages, sidebar
session items, and approval banners:
- overlay-slide-enter: settings and project settings panels
- overlay-backdrop-enter + overlay-panel-enter: modals
- message-enter: chat message fade-up on mount
- session-item-enter: sidebar item slide-in
- banner-slide-enter: approval banner slide-down

All animations use cubic-bezier(0.16, 1, 0.3, 1) for a snappy feel
and stay under 250ms. Respects prefers-reduced-motion.

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

src/renderer/components/ProjectSettingsPanel.tsx

@@ -111,7 +111,7 @@ export function ProjectSettingsPanel({
   }
 
   return (
-    <div className="fixed inset-0 z-50 flex flex-col bg-[var(--color-surface-0)]">
+    <div className="overlay-slide-enter fixed inset-0 z-50 flex flex-col bg-[var(--color-surface-0)]">
       {/* Header */}
       <div className="drag-region flex items-center gap-3 border-b border-[var(--color-border)] px-5 pb-3 pt-3">
         <button