Cancel a voting #6396

Problem & Kontext:

Currently, it is not possible to cancel an ongoing voting. The user can only click "End voting phase", which closes the voting and automatically evaluates the results. If a voting was started by mistake, or if the context changed and the vote is no longer relevant, hosts need a way to discard/abort the session entirely without generating results or affecting statistics.

User Story:

As a moderator,
i want to cancel an ongoing voting session without triggering an evaluation,
so that I can abort mistakenly started or invalid votings without saving or calculating results.

Acceptance Criteria:

Backend:

• Add the cancel state to the voting
• Update the voting update endpoint so that a voting can be canceled
• Ensure canceled votings do not trigger result evaluation logic or downstream notification triggers

Frontend & UX:

• Update the UI modal to include a "Cancel voting"-button (secondary) below "End voting phase" (primary)
• Implement frontend layout according to the final design specs (Design: TBD)