chore: remove dead per-type helpers in /f/[slug]/+page.svelte

After commit 10's ParticipantInput dispatch, the per-type helpers in the
participant page that the legacy inline rendering relied on are dead:

- toggleMultiChoice (only used by the old multi_choice render block)
- setDateRankedRating (only used by the old date_ranked_choice render block)
- dateRankedRating (only used by the old date_ranked_choice render block)

Removed. setAnswer is the one remaining helper — it's the callback wired
to ParticipantInput's setAnswer prop.

The three remaining `q.type === '...'` references in this file are
intentional locale customizations (boolean Ja/Nein label override and
date_ranked_choice per-option breakdown with formatted dates in the
previousSubmission summary view) — they don't fit the registry contract
and are flagged for m/fdbck#3 (i18n) when that ticket lands.

Verified post-Phase 2:
- 0 inline `q.type === '...'` strips outside lib/questions/ except the
  locale customizations above
- 0 errors, 25 warnings (pre-existing — same set as before Phase 2 plus
  one a11y warning in date_ranked_choice.builder.svelte that mirrors the
  legacy FormBuilder warning that's now gone)
- 123 server tests + 2 component tests pass
- bun run check + bun run build clean

src/routes/f/[slug]/+page.svelte

@@ -371,29 +371,10 @@
 		return dayBeforeFmt.format(d);
 	}
 
-	function toggleMultiChoice(q: FeedbackQuestion, opt: string): void {
-		if (q.type !== 'multi_choice') return;
-		const cur = (answers[q.id] as string[] | undefined) ?? [];
-		const next = cur.includes(opt) ? cur.filter((x) => x !== opt) : [...cur, opt];
-		answers = { ...answers, [q.id]: next };
-	}
-
 	function setAnswer(qid: string, value: unknown): void {
 		answers = { ...answers, [qid]: value };
 	}
 
-	function setDateRankedRating(qid: string, optId: string, rating: number | null): void {
-		const cur = (answers[qid] as Record<string, number | null> | undefined) ?? {};
-		answers = { ...answers, [qid]: { ...cur, [optId]: rating } };
-	}
-
-	function dateRankedRating(qid: string, optId: string): number | null {
-		const cur = answers[qid] as Record<string, number | null> | undefined;
-		if (!cur) return null;
-		const v = cur[optId];
-		return typeof v === 'number' ? v : null;
-	}
-
 	const dateOptionFmt = new Intl.DateTimeFormat([], {
 		weekday: 'short',
 		day: '2-digit',