feat(gui): add note history timeline with preview and restore

[haribo]

Summary

Exposes the note-history core (#183) in the GTK editor.

• A footer clock button opens an adw::Dialog Historique timeline: versions newest first, an Actuel chip on the current one, a > affordance on the rest.
• Selecting a version shows a read-only preview (title + body) with Restaurer / Retour.
• Restore applies the version forward (via commands::restore_version) and refreshes the open editor (title + buffer) without a spurious re-save, then reloads the sidebar.
• Time-only labels in v1 (the core exposes an opaque per-session author token, not a device name — per the validated mockup and design doc).

Test plan

• cargo clippy --workspace --all-targets -- -D warnings, cargo fmt --all --check
• PN_REQUIRE_GTK=1 cargo test -p plain-note-gui — green
• Golden re-recorded for the new footer button; run.sh verify = 0 pixel diff
• History dialog visually confirmed in the Docker+Xvfb harness (timeline + Actuel chip, light)

Committed with --no-verify (local .githooks/pre-commit broken, #186); staged content verified clean with gitleaks git --staged, CI Gitleaks re-checks.

Part of #179
Closes #189