Fix link context menus and temporary panel behavior

[aminought]

Right-clicking an empty page could show the sidebar link actions or leave an empty separator. Preview panels could also remain after closing because Firefox's hidden URL bar interrupted tab removal, and their buttons lost their colored background when Firefox renamed the attention color token.

This PR:

• Shows both link actions only for gContextMenu.onSaveableLink, ignores nested submenu events, and recomputes the native separator after updating visibility.
• Adds XULElement.toggleHidden(force) and documents the link actions in the README.
• Guards focus handling on the embedded window's uninitialized URL bar and avoids unloading a panel that closing has already removed.
• Restores temporary panel button backgrounds using the current attention color tokens, with legacy theme and system accent fallbacks.

Validation:

• npx eslint ., Prettier on all changed files, and git diff --check origin/master...HEAD passed.
• Firefox 155.0.1 on Windows: 13 headless menu/panel scenarios passed, including link/non-link contexts, separator transitions, temporary and permanent panels, button activation, and removal from the UI and saved panel settings.
• Light/dark color schemes and legacy/current/system color fallbacks passed. These behavioral checks ran on 1328a90, before the final toggleHidden helper refactor.
• Final commit 275c231 was installed in the test profile and passed the normal Firefox startup check; all 150 deployed source files matched the commit.