feat(ui): multi-select and bulk delete for timeline activity

[VooDisss]

Summary

This PR introduces a multi-selection mechanism for activity icons in the right sidebar timeline, enabling efficient bulk management of session history.

Changes

• Multi-Selection Logic:
  • CTRL+Click (or Cmd+Click) to toggle individual item selection.
  • SHIFT+Click for additive range selection from the last interacted item.
• Floating Action Pill:
  • Added a scroll-independent UI component at the bottom of the timeline strip.
  • Provides a "Delete" action with a dynamic selection count badge.
  • Includes a "Clear Selection" (X) button.
• Keyboard Support:
  • Delete / Backspace: Triggers the bulk-delete confirmation dialog for selected items.
  • Escape: Clears all current selections.
• UX Refinements:
  • Added 4rem bottom padding to the timeline to prevent the floating pill from obscuring history items.
  • Integrated visual highlights for selected badges using theme tokens.
  • Selection state is automatically cleared when loading a new session.

Related Issue

Closes #185

Verification Results

• Tested CTRL+Click toggling.
• Tested SHIFT+Click range selection (additive).
• Verified Delete and Esc keyboard shortcuts.
• Confirmed bulk deletion correctly removes all underlying message parts.
• Verified layout doesn't obscure badges at maximum scroll.

[VooDisss]

Works on mobile browser.