New send shortcut, Cancel that clears the draft, and a discard action for failed comments

This release refines how the comment input sends, cancels and recovers from a failed post. Some of these change the behavior of @beta components.

• Send shortcut: CommentInput and the other comment inputs send on Cmd+Enter, or Ctrl+Enter outside Apple platforms. Enter starts a new paragraph and Shift+Enter a new line. Neither sends while the mention menu is open or while an input method is composing text.
• Cancel: CommentInput always shows Cancel beside the submit control. Cancel empties the draft, including its stored copy, releases focus, so a composer with expandOnFocus folds back, and then calls onDiscard. Escape still calls onDiscard and keeps the draft, and so does an input unmounting, for example when the user switches threads. With confirmDiscard, both ask first. This applies to the reply box, the panel's new-thread composer, InlineCommentInput and the edit box in Comment, where Cancel drops the edited text and keeps the saved message.
• Failed comments: a comment that failed to post (state.type === 'createError') no longer gets the actions menu. Its only action is a discard button that calls onDelete without asking for confirmation. The new optional labels.actions.discard names it and defaults to "Discard". The notice's "Try again" still retries.

• Sending a message drops empty lines at either end, and the input clears to one empty line.
• The submit control shows a spinner while busy is set or while a submit is in flight.
• A selected CommentBadge uses the inline highlight's yellow instead of the gray button fill. In dark mode it uses the selected highlight's solid yellow with white text.
• Inline comment highlights fill exactly the line box at any text size and line height, so the bands of a wrapped run meet without overlapping.
• The list's loading text and empty text sit at the same height.