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/dashboard/mixins/specs/fileUploadMixin.spec.js

@@ -6,6 +6,8 @@ import { reactive } from 'vue';
 
 vi.mock('shared/helpers/FileHelper', () => ({
   checkFileSizeLimit: vi.fn(),
+  resolveMaximumFileUploadSize: vi.fn(value => Number(value) || 40),
+  DEFAULT_MAXIMUM_FILE_UPLOAD_SIZE: 40,
 }));
 
 vi.mock('activestorage', () => ({
@@ -27,6 +29,7 @@ describe('FileUploadMixin', () => {
   beforeEach(() => {
     mockGlobalConfig = reactive({
       directUploadsEnabled: true,
+      maximumFileUploadSize: 40,
     });
 
     mockCurrentChat = reactive({