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


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

---------

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

app/controllers/api/v1/widget/configs_controller.rb

@@ -9,7 +9,13 @@ class Api::V1::Widget::ConfigsController < Api::V1::Widget::BaseController
   private
 
   def set_global_config
-    @global_config = GlobalConfig.get('LOGO_THUMBNAIL', 'BRAND_NAME', 'WIDGET_BRAND_URL', 'INSTALLATION_NAME')
+    @global_config = GlobalConfig.get(
+      'LOGO_THUMBNAIL',
+      'BRAND_NAME',
+      'WIDGET_BRAND_URL',
+      'MAXIMUM_FILE_UPLOAD_SIZE',
+      'INSTALLATION_NAME'
+    )
   end
 
   def set_contact