fix: keep tab widths stable on hover

[16francej]

Overlay tab actions on a mostly opaque background so controls reveal without resizing tabs or reserving title space.

• 46 focused split, share, and archive tests pass.
• Web typechecks, lint, and formatting pass.
• 59 browser checks pass, including recovered title space, frame-by-frame geometry, narrow panes, keyboard sharing, and lone-tab controls.
• Six additional short/long-title and light/dark-theme cases pass.
• Synthetic-data preview checked under isolated browser restrictions.