feat: add batch unsubscribe UI to sidebar

[devwez]

Summary

Adds a visible multi-select mode indicator and batch action bar to the subscription column sidebar for easy multi-feed management.

Problem

Issue #5053 requests the ability to delete multiple RSS feeds at once. The batch unsubscribe functionality already exists (Cmd/Ctrl+Click to select, then right-click > Unfollow), but it is not discoverable - users have no visual indication that multi-select is possible.

Solution

1. Multi-select mode indicator: When feeds are selected, a checkbox icon appears in the header showing that multi-select mode is active. Clicking it exits the mode.

2. Batch action bar: When one or more feeds are selected, a bar slides up from the bottom of the sidebar showing:

   • Selection count (e.g., "3 feeds selected")
   • Cancel button to clear selection
   • "Unfollow All Selected Feeds" button with delete icon

3. i18n: Added translation keys for the new UI elements.

Changes

• SubscriptionColumnHeader.tsx - Added multi-select mode indicator button
• index.tsx - Added BatchActionBar component with selection count, cancel, and unfollow actions
• locales/app/en.json - Added translation keys

Testing

1. Hold Cmd/Ctrl and click multiple feeds in the sidebar
2. Verify the batch action bar appears at the bottom
3. Click "Unfollow All Selected Feeds" to confirm batch unsubscribe
4. Verify the multi-select mode indicator appears in the header
5. Click the indicator to exit multi-select mode