chore(branding): clean up product description strings

Strip the desktop package description to "Backspace" so Windows Task
Manager shows the bare product name instead of the long tagline.
Unify the web meta description and PWA manifest description on a single
positioning line that names Discord and TeamSpeak as the comparison
targets — improves link-preview copy and SEO surface.

packages/desktop/package.json

@@ -2,7 +2,7 @@
   "name": "@backspace/desktop",
   "version": "1.0.0",
   "private": true,
-  "description": "Backspace — a self-hosted, open-source chat platform",
+  "description": "Backspace",
   "author": {
     "name": "youruser",
     "email": "backspace@backspace.chat"