feat: allow configuring attachment upload limit (#12835)

## Summary
- add a configurable MAXIMUM_FILE_UPLOAD_SIZE installation setting and
surface it through super admin and global config payloads
- apply the configurable limit to attachment validations and shared
upload helpers on dashboard and widget
- introduce a reusable helper with unit tests for parsing the limit and
extend attachment specs for configurability


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[Codex
Task](https://chatgpt.com/codex/tasks/task_e_6912644786b08326bc8dee9401af6d0a)

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Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: iamsivin <iamsivin@gmail.com>

app/javascript/shared/constants/messages.js

@@ -34,9 +34,6 @@ export const CONVERSATION_PRIORITY_ORDER = {
   low: 1,
 };
 
-// Size in mega bytes
-export const MAXIMUM_FILE_UPLOAD_SIZE = 40;
-
 export const ALLOWED_FILE_TYPES =
   'image/*,' +
   'audio/*,' +