feat: add visited-page navigation history

[emsi]

Summary

• add bounded Back/Forward page history across open notebooks using stable source, section, and page identifiers
• expose history through the application menu, Alt+Left/Alt+Right, standard Back/Forward key events, and conventional Linux mouse navigation buttons
• keep ordinary pointer events untouched through a dedicated raw input adapter and place initial focus on non-destructive notebook navigation
• document the navigation contract and add deterministic state tests plus Xvfb/xdotool pointer-propagation coverage

Verification

• cargo test --workspace --all-targets --locked --offline
• cargo clippy --workspace --all-targets --locked --offline -- -D warnings
• GTK/Xvfb viewer, renderer, icon, and GUI smoke tests
• dependency license audit
• GitHub Actions release-build run 31318741537: native, Flatpak, and AppImage jobs passed
• Flatpak manually verified by the project owner

Closes #33