Add podcast playback, Fountain.fm cards, V4V streaming, fix notifications

Podcast feature:
- Podcast discovery via Podcast Index API (trending + search)
- Persistent player bar with play/pause, seek, speed (1x/1.5x/2x), volume
- Audio persists across view navigation, resumes from saved position
- Fountain.fm URL detection in feed with rich playable cards
- "Play in Wrystr" button on inline audio blocks
- V4V streaming sats via NWC (LNURL-pay, 5min accumulation, split payments)
- Share what you're listening to (publish note with confirm)
- Space key toggles play/pause globally

Notification fixes:
- Per-notification read tracking (click to mark read) instead of mark-all-on-open
- Read notifications persist at 50% opacity, unread get accent border
- Always fetches last 7 days, keeps 15 most recent
- Filter out own replies from notifications
- Sidebar badge shows only unread count

src/components/sidebar/Sidebar.tsx

@@ -12,6 +12,7 @@ const NAV_ITEMS = [
   { id: "feed" as const, label: "feed", icon: "◈" },
   { id: "articles" as const, label: "articles", icon: "☰" },
   { id: "media" as const, label: "media", icon: "▶" },
+  { id: "podcasts" as const, label: "podcasts", icon: "P" },
   { id: "search" as const, label: "search", icon: "⌕" },
   { id: "bookmarks" as const, label: "bookmarks", icon: "▪" },
   { id: "dm" as const, label: "messages", icon: "✉" },