Rename the project to ihasvpn

WGX shares its name with several other WireGuard tools, so the project
becomes ihasvpn, alongside ihasmail.

- Module github.com/Coffey-Labs/ihasvpn, command cmd/ihasvpn, image
  ghcr.io/coffey-labs/ihasvpn.
- Environment variables move from WGX_* to IHASVPN_*. The default database
  is ihasvpn.db, the nftables table is `ihasvpn`, metrics are ihasvpn_*, and
  the session cookie and theme key are renamed, so existing sessions end.
- The mark is the ihasmail cat peeking over the edge of a shield, drawn as
  a vector. docs/brand/generate.py builds the mark, mono mark, wordmarks,
  social card, favicons and app icons from that one drawing.
- The console takes ihasmail's palette: the ihasmail.org teal-navy for dark,
  its contrast-checked light tiers with the site's light accent, received
  traffic in the cat's orange and sent in teal. The wordmark weight and
  font stack follow ihasmail.org.
- Detail values wrap at spaces before breaking inside an address, so an
  IPv6 tunnel address no longer splits mid-number.
- The README history note about the earlier WGX installer is gone with the
  name it explained. Screenshots retaken.

web/public/site.webmanifest

@@ -1,11 +1,11 @@
 {
-  "name": "WGX",
-  "short_name": "WGX",
-  "description": "WireGuard server console",
+  "name": "ihasvpn",
+  "short_name": "ihasvpn",
+  "description": "Self-hosted WireGuard server",
   "start_url": "/",
   "display": "standalone",
-  "background_color": "#111715",
-  "theme_color": "#121a17",
+  "background_color": "#0d2430",
+  "theme_color": "#0d2430",
   "icons": [
     { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png" },
     { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png" }