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/helpers/FileHelper.js

@@ -1,3 +1,5 @@
+export const DEFAULT_MAXIMUM_FILE_UPLOAD_SIZE = 40;
+
 export const formatBytes = (bytes, decimals = 2) => {
   if (bytes === 0) return '0 Bytes';
 
@@ -19,3 +21,13 @@ export const checkFileSizeLimit = (file, maximumUploadLimit) => {
   const fileSizeInMB = fileSizeInMegaBytes(fileSize);
   return fileSizeInMB <= maximumUploadLimit;
 };
+
+export const resolveMaximumFileUploadSize = value => {
+  const parsedValue = Number(value);
+
+  if (!Number.isFinite(parsedValue) || parsedValue <= 0) {
+    return DEFAULT_MAXIMUM_FILE_UPLOAD_SIZE;
+  }
+
+  return parsedValue;
+};