feat: add context menu with delete confirmation for sidebar notes

[j4rviscmd]

Summary

• Add right-click context menu on sidebar note items with a "Delete" option
• Show confirmation dialog (AlertDialog) before deletion to prevent accidental data loss
• Auto-select the most recent remaining note after deletion to avoid empty state on restart

Changes

• Install context-menu and alert-dialog shadcn/ui components
• Add onDeleteNote prop to NoteSidebar with confirmation flow
• Add handleDeleteNote callback in App.tsx that selects the next recent note
• Simplify formatDate and groupNotes helper functions

Test plan

• Right-click a note in the sidebar → context menu appears with "Delete" option
• Click "Delete" → confirmation dialog shows
• Confirm deletion → note is removed, most recent note is auto-selected
• Cancel deletion → nothing happens
• Delete the currently selected note → next recent note is selected
• Delete the only remaining note → no note is selected (empty state)

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