Multi-Select & Bulk Actions in Sidebar

> ℹ️ _This issue was partially AI-generated and may contain technical inaccuracies._

**Is your feature request related to a problem? Please describe.**
Sidebar actions operate on a single item at a time. Moving, deleting, or duplicating many requests/folders is tedious.

**Describe the solution you'd like**
Support multi-selecting requests/folders in the sidebar (ctrl/shift-click) and applying bulk actions: move, delete, duplicate. Integrate with the existing drag-and-drop reordering (#414).

**Describe alternatives you've considered**
Repeating single-item actions; doing bulk file operations outside the app.

**Additional context**
Builds on the existing sidebar and dnd-kit reordering. Renderer-only for the most part.

Start by reading the existing sidebar implementation and the drag-and-drop reordering work referenced in issue #414, including its dnd-kit integration. Trace how requests and folders are selected and how move, delete, and duplicate actions currently operate. Done means ctrl/shift multi-selection supports those bulk actions without breaking drag-and-drop reordering.

Written by the indexing model from the issue text.