Fix README limit claims to match reality (issue #13)

- 'No file limit' -> 100 MB default, configurable via max_upload_bytes
- 'No character limit' -> 'No 4,096-character limit like Telegram'
  (hard frame-body cap: 1 MiB)
- Note that limits are set on the gateway side (hermes), not in the app
- Sync docs/playstore-listing.md (same overclaim)

docs/playstore-listing.md

@@ -23,8 +23,9 @@ threads, media, search — Telegram-quality, on your own infrastructure.
   **Google's servers**. If you want truly private communication, use ntfy
   (self-hosted) instead — it is the default.
 
-No file limit. No character limit. Full markdown + HTML artifact preview.
-All settings live in the app.
+100 MB file uploads by default (configurable on your gateway). No
+4,096-character message limit like Telegram. Full markdown + HTML artifact
+preview. All settings live in the app.
 
 Runs on Android and desktop (Linux, macOS, Windows) from one shared codebase.