Fix feed OOM: lazy image loading, inView gating, WebKit memory tuning

- NoteContent: remove ImageGrid from inline mode — images now only load
  when note is inView (via mediaOnly), stopping runaway scrolling leak
- NoteCard: content-visibility:auto to skip layout/paint for off-screen
  cards; inView-gated media, NoteActions, NIP-05 verification
- useInView: new IntersectionObserver hook with 300px rootMargin
- useProfile: MAX_PROFILE_CONCURRENT=8 throttle with fetch queue
- useReplyCount/useZapCount/useReactions: enabled param, throttled queues
- feed.ts: MAX_FEED_SIZE 200→30, live sub disabled (pendingNotes pattern),
  250ms batch debounce on live events
- core.ts: MAX_CONCURRENT_FETCHES=25 global NDK cap, fetchWithTimeout
  uses subscribe+stop instead of fetchEvents (no zombie subscriptions)
- lib.rs: HardwareAccelerationPolicy::Never + CacheModel::DocumentViewer
- main.rs: WEBKIT_DISABLE_COMPOSITING_MODE=1 for Linux
- relay/db.rs: TTL eviction + 5000 event cap
- feedDiagnostics.ts: file-flushing diag log survives crashes

src-tauri/capabilities/default.json

@@ -27,6 +27,12 @@
         { "path": "**" }
       ]
     },
+    {
+      "identifier": "fs:allow-write-text-file",
+      "allow": [
+        { "path": "$HOME/vega-diag.log" }
+      ]
+    },
     "notification:default"
   ]
 }