Added ability to multi-select torrents.

[himsin]

Desktop UI:

1. Checkbox toggle without Ctrl: The checkbox in each row now has its own onClick handler that toggles selection independently of the row click. Clicking the checkbox adds/removes the torrent from selection without needing Ctrl. Clicking elsewhere on the row still works with the existing single/Ctrl/Shift selection logic.
2. Multi-select detail panel: When multiple torrents are selected, the details panel shows "N torrents selected" instead of "Select a torrent". Single-torrent selection still shows the full detail view (existing behavior preserved).

Mobile UI:

1. Long-press to multiselect: A 500ms touch-hold on any torrent card enters multiselect mode and selects that torrent. Includes haptic feedback.
2. Tap to toggle in multiselect mode: When multiselect is active, tapping a torrent toggles its selection instead of opening the detail drawer. If all torrents are deselected, multiselect mode automatically turns off.
3. Visual selection indicators: Selected cards get an accent-colored border/background. In expanded mode, the state icon is replaced with a checkmark circle. In compact mode, a checkbox circle appears before the name.
4. Floating action toolbar: When in multiselect mode, a fixed toolbar appears above the bottom navigation with:
   4.1. X button to cancel multiselect
   4.2. Selection count display
   4.3. Play (start/resume), Square (stop), Trash (delete) action buttons
   4.4. All actions are grouped by instance and applied to all selected torrents
5. Delete confirmation modal: Delete shows a confirmation dialog with "Remove from list" and "Delete with files" options, matching the desktop delete modal pattern.
6. Swipe disabled in multiselect: The swipe-to-toggle gesture is hidden during multiselect mode to avoid conflicts.