Hide Vote button and show banner when browser voting is disabled

[ArendPeter]

Parent

Part of #1603.

What to build

When online (browser) voting isn't an allowed submission channel for an election, voters see a clear banner in place of being able to cast a ballot, and the "Vote" button is hidden from the election's home page — the same treatment already used for draft/archived elections.

Acceptance criteria

• The election home page hides the "Vote" button when submitted_via_browser isn't in the resolved allowed_submit_types (election.settings.allowed_submit_types ?? DEFAULT_ALLOWED_SUBMIT_TYPES) — same treatment as an archived election.
• The vote page shows a banner ("Online voting is not enabled for this election. Please contact the election administrator.") in that case, styled and placed like the existing draft/archived banners (alongside the ballot, not replacing the page).
• Test coverage (unit or E2E) for both the hidden-button and banner states, driven by setting allowed_submit_types to exclude submitted_via_browser on a fixture/test election.

Blocked by

#1604 (needs the allowed_submit_types field and default to exist).