feat(share): branded question share card with share analytics

Replace the inline ShareableCard in SharePreviewModal with a designed
QuestionShareCard: Sauci logo + wordmark header, pack badge, and a
branded sauci.app footer pill on the captured story image. The card
deliberately never carries the couple join code - it is a private
pairing credential, not a referral link (a regression test enforces
this). Text-share fallbacks now carry a full https://sauci.app link,
and all three share paths fire a new question_shared analytics event
with the destination.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/mobile/src/lib/analytics.ts

@@ -214,6 +214,8 @@ export const Events = {
   questionAnswered: (answer: string, packId?: string) =>
     logEvent("question_answered", { answer, pack_id: packId ?? "unknown" }),
   questionSkipped: () => logEvent("question_skipped"),
+  questionShared: (destination: "instagram" | "messages" | "more") =>
+    logEvent("question_shared", { destination }),
   matchCreated: (matchType: string) => logEvent("match_created", { match_type: matchType }),
   allQuestionsExhausted: () => logEvent("all_questions_exhausted"),
   pendingQuestionsPromptViewed: (count: number) =>