fix: add explicit delete cancel and clean up cell controls

[paolino]

Summary

• Adds explicit cancel affordances for every armed delete surface, plus Escape, click-away, and stale-safe 2800ms auto-cancel handling.
• Centers icon glyphs in their button boxes and replaces the pause-like grip glyph with a six-dot drag handle.
• Replaces the cell eye select button with per-track radio selectors wired to the existing selectCell action.

Validation

• nix develop .#frontend --command spago test
• ./gate.sh (build, unit, format-check, hlint, frontend Nix build, browser interaction smoke)

Closes #34