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>

config/installation_config.yml

@@ -84,6 +84,11 @@
   value: false
   description: 'Enable direct uploads to cloud storage'
   locked: false
+- name: MAXIMUM_FILE_UPLOAD_SIZE
+  value: 40
+  display_title: 'Attachment size limit (MB)'
+  description: 'Maximum attachment size in MB allowed for uploads'
+  locked: false
 # ------- End of Account Related Config ------- #
 
 # ------- Email Related Config ------- #