docs: add repository cover and avatar images

Add a 1280x640 cover shown at the top of the README, a square variant and
a text-free repository avatar that stays legible at small sizes. SVG
sources are kept next to the rendered PNGs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -1,5 +1,7 @@
 # Balance Server
 
+![Balance Server — self-hosted sync and accounts for Balance](docs/cover.png)
+
 Self-hosted synchronization, account and web server for the Balance iOS, macOS, watchOS and Android applications. It is a single Rust process with a bundled SQLite engine and a complete browser client, and runs on Linux, Windows and macOS.
 
 ## Quick start with Docker