Confirm before converting tasks and headings to projects

[MaplumeX]

Summary

Converting a task or a project heading into a project is destructive and irreversible — the source is physically deleted (ADR-0024). Previously a task with no attachments converted immediately with no confirmation, and a heading converted straight from the actions menu with no warning about its tasks being moved. This adds an explicit confirmation step to both paths.

Changes

• Task conversion (useConvertGuard): always show a confirmation dialog instead of skipping it when there are no attachments. The copy now branches — the attachment-specific warning (convertWithAttachments, ADR-0019) when attachments exist, otherwise the new generic convertConfirm text. The confirm button stays destructive only when attachments will be deleted, otherwise default. Keeps the last attachment count in a ref so the description doesn't flicker back during the close animation.
• Heading conversion (ProjectHeadingRow): selecting "Convert to Project" now opens a confirmation dialog (convertHeadingConfirm) explaining that the heading becomes a new project and its tasks move into it. The mutation only runs after confirming.
• Added convertConfirm and convertHeadingConfirm strings for both en and zh.

Testing

• Updated ProjectHeadingRow.test.tsx to assert the mutation is not called on menu select and is called after confirming in the dialog.