fix: use alerts API and bind upload/picture handlers to active composer (NodeBB 4.4.x)

[MusikschuleFroehlich]

Summary

• Use NodeBB’s alerts module instead of app.alert/app.removeAlert/app.alertError.
• Bind “picture” and “upload” toolbar handlers to the active composer container to ensure file input opens in the correct instance.
• Remove Quill-side upload toasts to avoid duplicate notifications (global uploadHelpers already shows them).
  Problem
• Upload toasts were using app.alert, which is not available in recent NodeBB.
• Upload/picture handlers could be bound outside the active composer, causing file input to target the wrong container.
• Duplicate upload toasts (Quill + global uploadHelpers).
  Solution
• Replace app.alert* with alerts.alert/alerts.remove/alerts.error.
• Execute toolbar handlers in the context of the current postContainer (composer instance).
• Do not emit Quill-specific upload toasts; rely on global uploadHelpers.
  Changes
• static/lib/quill-nbb.js only (client-side).
  Compatibility
• Tested on NodeBB 4.4.6, default theme.
  Test plan
• Start NodeBB in development mode with unminified assets.
• Open composer (new topic and reply), verify:
• Drag & drop and button-initiated upload both succeed (2xx on POST /api/post/upload).
• No duplicate toasts
• Uploaded images embed at cursor, files insert as links.
• Open two composers in parallel; verify each upload targets the correct instance.
  Related issues
• Fixes Upload and Picture handlers should be bound to the current editor #71
• Relates to general upload UX consistency (duplicate toasts).