Restore submit button double-click guard on summary page

[nayij]

The prevent-multiple-clicks class added in #413 was dropped when the i18n-linting branch (#436) was merged, leaving the summary submit button with preventDoubleClick: false and no client-side guard at all. Users clicking Submit repeatedly on a slow submission (e.g. many file uploads) could trigger duplicate form submissions.

Re-add the class so debounce-click.js ignores repeat clicks for 10s, and add a test that renders the real summary page, runs initDebounceClick() against it and asserts a double click only submits the form once.

Proposed change

Jira ticket:

Type of change

• Bug fix
• New feature
• Breaking change
• Misc. (documentation, build updates, etc)

Checklist

• You have executed this code locally and it performs as expected.
• You have added tests to verify your code works.
• You have added code comments and JSDoc, where appropriate.
• There is no commented-out code.
• You have added developer docs in README.md and docs/* (where appropriate, e.g. new features).
• The tests are passing (npm run test).
• The linting checks are passing (npm run lint).
• The code has been formatted (npm run format).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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