feat(ui): mention tokens render as chips in the comment composer; ui 0.44.0 (#1577)

When a host supplies mentionSource, the @Label tokens the picker inserted render as chips inside the composer. The composer's existing skill-reference overlay is generalized into one token highlight layer fed by merged ranges from the new pure utils/composerTokens module; chips follow the picked-id model, carry data-mention-token and data-mention-kind, take an optional tokenClassName, and change paint only so the caret never drifts. With skill references only the overlay is byte-identical to before, and with neither source no overlay element exists.