[Capture recovery] Preserve comma-containing labels in the capture composer

[Chris0Jeky]

Context

Late review on parked PR #2047 identified a remaining composer-only lossiness seam. PaperCaptureComposer currently treats a comma key as a label delimiter. Typing an existing board label such as Sales, EMEA therefore produces separate Sales and EMEA chips, and the metadata request persists those split names. Triage then rejects them because neither split name exists on the selected board.

Review thread: #2047 (comment)

This is separate from the correction editor's lossless chip behavior and is non-blocking for the parked PR's normal-path proof.

Acceptance criteria

• The capture composer can select or enter an existing comma-containing label without splitting it.
• Label commit uses an explicit lossless action such as Enter/Add or a board-label selector; comma is ordinary label content.
• The create-capture request contains one exact label name for Sales, EMEA.
• Focused tests cover composer interaction and the emitted metadata payload.
• The recoverable unknown-label path and review-before-apply boundary remain unchanged.

Boundary

• Track as a fresh successor slice; this issue does not authorize another push to parked PR Recover stranded capture metadata #2047.
• Use synthetic labels only in manual or browser evidence.