Fix blank recurring page, avatar crash, loading skeleton, and empty state

[oluebubejoy]

Fixes #957, Fixes #958, Fixes #959, Fixes #960

What changed

• [Frontend] recurring/page.tsx shows a completely blank page when every subscription is cancelled #957: Compute hasAny from only activeSubscriptions.length + pausedSubscriptions.length, not the full subscriptions array — prevents blank page when all subscriptions are cancelled
• [Frontend] Explore page and header search crash on an empty display name — same crash already fixed in ProfileCard, left open in two other files #958: Guard avatar fallback with optional chaining and || '?' in explore/page.tsx and app-shell.tsx, matching the safe pattern already in profile-card.tsx
• [Frontend] NotificationPreferences renders nothing (not even a spinner) while loading, leaving a blank gap on Settings #959: Render a skeleton placeholder in NotificationPreferences while loading instead of returning null, eliminating the blank gap on the Settings page
• [Frontend] Support-history tab always shows a contradictory "No transactions yet" message even when the creator has transactions and a search/filter just matched zero results #960: Gate the EmptyState block on transactions.length === 0 so filtered-empty searches show only the "no matches" message, not a contradictory "No transactions yet"

Why

• [Frontend] recurring/page.tsx shows a completely blank page when every subscription is cancelled #957: Users with only cancelled subscriptions see an empty page with no explanation
• [Frontend] Explore page and header search crash on an empty display name — same crash already fixed in ProfileCard, left open in two other files #958: Empty displayName crashes the Explore grid and search dropdown
• [Frontend] NotificationPreferences renders nothing (not even a spinner) while loading, leaving a blank gap on Settings #959: Blank strip visible between Settings sections while data loads
• [Frontend] Support-history tab always shows a contradictory "No transactions yet" message even when the creator has transactions and a search/filter just matched zero results #960: Two stacked contradictory empty-state messages when filters match nothing

How to test

• Verify recurring page shows "No recurring support yet" when all subscriptions are cancelled
• Verify profiles with empty displayName render ? fallback instead of crashing
• Verify notification preferences show skeleton rows while loading
• Verify support-history tab shows only "no matches" when filters match nothing, not "No transactions yet"

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