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/web/server/index.js

@@ -12529,6 +12529,46 @@ async function main(options = {}) {
     }
   });
 
+  app.post('/api/git/worktrees/preview', async (req, res) => {
+    const { previewWorktreeCreate } = await getGitLibraries();
+    if (typeof previewWorktreeCreate !== 'function') {
+      return res.status(501).json({ error: 'Worktree preview is not available' });
+    }
+
+    try {
+      const directory = req.query.directory;
+      if (!directory || typeof directory !== 'string') {
+        return res.status(400).json({ error: 'directory parameter is required' });
+      }
+
+      const preview = await previewWorktreeCreate(directory, req.body || {});
+      res.json(preview);
+    } catch (error) {
+      console.error('Failed to preview worktree:', error);
+      res.status(500).json({ error: error.message || 'Failed to preview worktree' });
+    }
+  });
+
+  app.get('/api/git/worktrees/bootstrap-status', async (req, res) => {
+    const { getWorktreeBootstrapStatus } = await getGitLibraries();
+    if (typeof getWorktreeBootstrapStatus !== 'function') {
+      return res.status(501).json({ error: 'Worktree bootstrap status is not available' });
+    }
+
+    try {
+      const directory = req.query.directory;
+      if (!directory || typeof directory !== 'string') {
+        return res.status(400).json({ error: 'directory parameter is required' });
+      }
+
+      const status = await getWorktreeBootstrapStatus(directory);
+      res.json(status);
+    } catch (error) {
+      console.error('Failed to get worktree bootstrap status:', error);
+      res.status(500).json({ error: error.message || 'Failed to get worktree bootstrap status' });
+    }
+  });
+
   app.delete('/api/git/worktrees', async (req, res) => {
     const { removeWorktree } = await getGitLibraries();
     if (typeof removeWorktree !== 'function') {