feat(ui): add bulk memo selection

[BrenticusMaximus]

Summary

• Add “Select” to memo action menu
• Enable multi‑select with checkboxes and selection highlighting
• Provide bulk pin/archive/delete actions in a sticky header bar (desktop + mobile)
• Add confirmation for bulk delete and success toasts

Motivation

Make it easy to manage multiple memos at once in a private/single‑user workflow.

Testing

• Open memo menu → Select
• Select multiple memos
• Bulk pin/archive/delete from header bar
• Confirm delete dialog works