add note history (timeline + restore) across core and clients

[haribo]

Per-note version timeline with restore. Full design: docs/design/note-history.md.

Why

Notes are Automerge CRDT docs that already retain every change (used for sync),
so a history/restore feature needs no new stored data, no wire change, and does
not affect the zero-knowledge relay — it is a read-only derivation done on-device.

Locked decisions

• Version boundary = device change OR ≥ 5 min idle gap between changes.
• Zero-diff dedup only (drop a boundary only if its content equals the previous
  kept version). Never drop for "small" diffs.
• Retention: 100 most recent versions per note — a display/derivation cap, NOT a
  storage bound. Underlying ops are kept, so the cap is reversible.
• Scope of a version: title, body, tags, folder. Restore replaces the whole set
  as a forward, merge-safe change (itself a new history entry).

Sub-issues

• document the note-history design (docs/design/note-history.md)
• implement note history in core (note_history / note_at / restore) + tests
• add pn history / --at / restore to the CLI
• add a history timeline + restore to the GUI
• add a history screen + restore to Android (mockup-first)

Out of scope

Operation compaction/pruning (own ADR), diff/blame view, per-field or partial
restore, branching/named versions, history search.

[haribo]

All sub-issues merged into develop: design (#181), core (#183), CLI (#188), GUI (#190), Android (#193). Note history (timeline + preview + restore) is complete across all three clients.