Implement Draft Vote Feature (api-draft-vote)

[tran2106]

• Add a new route: PATCH /api/polls/:pollId/vote to save a user's ballot as a draft (not final submission).
• Allow logged-in users to save incomplete rankings and return later to finish.
• Update the vote model to support a submitted flag (false for drafts, true for final submission).
• Ensure users can only have one draft per poll.
• Add frontend support to show a warning for incomplete rankings and allow saving as draft.
• Show confirmation after successful draft save or final submission.