* feat: Bulk Delete Chats from Sidebar
feat(sidebar): Implement bulk chat deletion
Adds the ability for users to select multiple chats from the history sidebar and delete them in bulk.
**Key Changes:**
* **Selection Mode:** Introduced a selection mode toggled by a dedicated button next to "Start new chat".
* **Checkboxes:** Added checkboxes to each `HistoryItem` visible only when selection mode is active.
* **Bulk Actions:** Added "Select All" / "Deselect All" and "Delete Selected" buttons (`Button` component with `ghost` variant) that appear above the chat list in selection mode.
* **Confirmation Dialog:** Implemented a confirmation dialog (`Dialog` component) to prevent accidental deletion, listing the chats selected for removal.
* **Deletion Logic:** Updated `Menu.client.tsx` to handle the selection state and perform bulk deletion using `deleteById` from persistence layer.
* **Styling:** Ensured all new UI elements (`Checkbox`, `Button`) adhere to the existing project design system and support both light and dark themes using appropriate CSS classes and UnoCSS icons (`i-ph:` prefix).
* **Refinement:** Replaced initial plain `<button>` elements with the project's `Button` component for consistency. Fixed incorrect icon prefixes.
* Fix selection and Dark mode
