Add durable user-message queue dispatch

[copyberry]

Add durable user-message queue dispatch

What changed

• Add a storage-neutral queue extension for listing, adding, editing, reordering, deleting, and explicitly starting queued user messages.
• Dispatch queued messages in FIFO order when a thread becomes idle after a completed turn, while leaving them queued after interruptions or failures.
• Wait for user input to be persisted before removing its queue item, and report hook rejection, task termination, and persistence failures distinctly.
• Snapshot local image and audio attachments into validated data URLs before storing them, enforce input limits, and emit ThreadQueueChanged events after mutations.

Testing

• Cover queue mutations, automatic and explicit dispatch, interruption and failure behavior, hook rejection, invalid records, input limits, and local attachment snapshotting.