feat: allow message block edition

[clementb49]

Summary by CodeRabbit

• New Features

  • Introduced an editing dialog for message blocks, allowing users to modify prompts, system messages, assistant responses, models, and attachments.
  • Added a unified prompt and attachment panel with improved file, image, and URL handling, plus keyboard shortcuts and context menus.
  • Enhanced completion handling with centralized streaming and non-streaming support, including callbacks and accessibility improvements.
  • Added a new accessible output handler for improved speech and braille output with streaming support.

• Improvements

  • Streamlined accessible output and speech streaming for better accessibility and error handling.
  • Modularized prompt, attachment, and completion logic for a more consistent and user-friendly experience.
  • Improved attachment validation and compatibility checks with AI models.
  • Updated conversation and engine logic to support partial message processing by index for more flexible completions.
  • Simplified attachment management by delegating UI interactions to the prompt attachments panel.
  • Enhanced UI responsiveness by handling URL attachments asynchronously.

• Bug Fixes

  • Fixed attachment and prompt handling in various UI components to ensure consistent behavior and validation.

• Refactor

  • Simplified shutdown and completion stopping procedures.
  • Removed direct threading and UI management from conversation tab, delegating to centralized handlers and panels.
  • Replaced manual accessible output handling with a unified handler across components.
  • Redirected attachment-related operations to the new prompt attachments panel for consistency.