feat: add ability to edit queued messages with pause for top item

[1code-async]

Summary

• Adds the ability to edit queued messages before they are sent
• Automatically pauses queue processing when editing the first item in the queue to prevent the message from being sent while being modified
• Resumes processing automatically when the edit dialog is closed (save or cancel)

Changes

• message-queue-store.ts: Added updateQueueItem, setEditingItem, and isItemEditing functions to track and update queue items being edited
• edit-queued-message-dialog.tsx: New dialog component for editing queued message text with keyboard shortcuts (Cmd/Ctrl+Enter to save, Escape to cancel)
• queue-processor.tsx: Added logic to pause processing when the first queue item is being edited, with subscription to editing state changes
• agent-queue-indicator.tsx: Added edit button (pencil icon) to each queue item row
• active-chat.tsx: Passed subChatId prop to AgentQueueIndicator for edit dialog context

Test plan

• Queue multiple messages while the agent is streaming
• Click edit icon on a queued message to open edit dialog
• Verify that editing the first item in queue shows "Queue processing is paused while editing" message
• Verify that editing items not at the front of the queue shows "Edit the message before it's sent" message
• Make changes and click Save - verify the queued message is updated
• Click Cancel or press Escape - verify no changes are made
• Verify queue processing resumes after closing the edit dialog

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