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/lib/gitApiHttp.ts

@@ -424,6 +424,30 @@ export async function validateGitWorktree(directory: string, payload: CreateGitW
   return response.json();
 }
 
+export async function getGitWorktreeBootstrapStatus(directory: string): Promise<import('./api/types').GitWorktreeBootstrapStatus> {
+  const response = await fetch(buildUrl(`${API_BASE}/worktrees/bootstrap-status`, directory));
+  if (!response.ok) {
+    const error = await response.json().catch(() => ({ error: response.statusText }));
+    throw new Error(error.error || 'Failed to get worktree bootstrap status');
+  }
+  return response.json();
+}
+
+export async function previewGitWorktree(directory: string, payload: CreateGitWorktreePayload): Promise<GitWorktreeCreateResult> {
+  const response = await fetch(buildUrl(`${API_BASE}/worktrees/preview`, directory), {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(payload ?? {}),
+  });
+
+  if (!response.ok) {
+    const error = await response.json().catch(() => ({ error: response.statusText }));
+    throw new Error(error.error || 'Failed to preview worktree');
+  }
+
+  return response.json();
+}
+
 export async function createGitWorktree(directory: string, payload: CreateGitWorktreePayload): Promise<GitWorktreeCreateResult> {
   const response = await fetch(buildUrl(`${API_BASE}/worktrees`, directory), {
     method: 'POST',