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/types/podcast.ts

@@ -0,0 +1,33 @@
+export interface PodcastShow {
+  feedUrl: string;
+  title: string;
+  author: string;
+  artworkUrl: string;
+  description: string;
+  podcastIndexId?: number;
+}
+
+export interface V4VRecipient {
+  name?: string;
+  type: string;
+  address: string;
+  split: number;
+  customKey?: string;
+  customValue?: string;
+}
+
+export interface PodcastEpisode {
+  guid: string;
+  title: string;
+  enclosureUrl: string;
+  pubDate: number;
+  duration: number;
+  description: string;
+  artworkUrl?: string;
+  showTitle: string;
+  showArtworkUrl: string;
+  podcastIndexId?: number;
+  value?: V4VRecipient[];
+}
+
+export type PlaybackState = "idle" | "loading" | "playing" | "paused";