Make page header text selectable

[emsi]

Summary

• make the page title and full notebook/section context selectable with native GTK label behavior
• preserve the complete Unicode value when copying an ellipsized header
• run the complete viewer test suite under Xvfb in release CI
• document page-header selection separately from future freeform canvas selection

Validation

• cargo fmt --all -- --check
• cargo test --workspace --all-targets --locked --offline
• cargo clippy --workspace --all-targets --locked --offline -- -D warnings
• GTK_A11Y=none GSK_RENDERER=cairo xvfb-run -a cargo test -p onenote-viewer --locked --offline
• git diff --check

Closes #19