feat: instant draft-first worktree creation and multi-run launcher redesign (#741)

## Summary

- **Instant worktree creation from chat draft**: selecting "+ New worktree" in the draft branch selector immediately creates a session draft and bootstraps the worktree in the background — no modal interruption
- **Redesigned multi-run launcher**: compact 2-column grid layout in a right-sized dialog with scroll shadow, sticky footer, tooltips replacing verbose descriptions, and project icons in the selector
- **Branch selector aligned across surfaces**: multi-run and agent manager branch pickers now use the shared git store and match NewWorktreeDialog behavior (same default resolution cascade, no synthetic HEAD option, all branches shown)
- **Opaque model multi-select dropdown**: fixes text bleed-through on translucent backgrounds by compositing `--surface-elevated` over `--surface-background`
- **"+ New" inline button in sidebar worktree headers** for faster worktree creation

## Why

Worktree creation was behind modal flow that interrupted the user's train of thought. The draft-first approach lets users start typing immediately while the worktree bootstraps. The multi-run launcher had an oversized form layout with redundant explanations, and its branch picker behaved differently from the main worktree dialog - causing confusion about which branches were available and what the default was.

packages/ui/src/components/views/SettingsWindow.tsx

@@ -30,18 +30,13 @@ export const SettingsWindow: React.FC<SettingsWindowProps> = ({ open, onOpenChan
       <DialogPrimitive.Portal>
         <DialogPrimitive.Overlay
           className="fixed inset-0 z-50 bg-black/50 backdrop-blur-md"
-          onPointerDown={(event) => {
-            event.stopPropagation();
-            if (hasOpenFloatingMenu()) {
-              return;
-            }
-            onOpenChange(false);
-          }}
         />
         <DialogPrimitive.Content
           aria-describedby={descriptionId}
-          onPointerDownOutside={(event) => {
-            event.preventDefault();
+          onInteractOutside={(event) => {
+            if (hasOpenFloatingMenu()) {
+              event.preventDefault();
+            }
           }}
           className={cn(
             'fixed z-50 top-[50%] left-[50%] translate-x-[-50%] translate-y-[-50%]',