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/ChatPane.tsx

@@ -345,7 +345,7 @@ export function ChatPane({
                   phase === 'thinking' ? 'Thinking' : phase === 'final' ? 'Final' : undefined;
 
                 return (
-                  <div className="group py-3" data-message-id={message.id} key={message.id}>
+                  <div className="message-enter group py-3" data-message-id={message.id} key={message.id}>
                     <div className="flex gap-3">
                       <div
                         className={`mt-0.5 flex size-7 shrink-0 items-center justify-center rounded-full ${
@@ -448,7 +448,7 @@ export function ChatPane({
         <div className="mx-auto max-w-3xl">
           {/* Pending approval banner */}
           {pendingApproval && (
-            <div className="mb-3 space-y-2">
+            <div className="banner-slide-enter mb-3 space-y-2">
               <ApprovalBanner
                 approval={pendingApproval}
                 isResolving={isResolvingApproval}