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.

internal/auth/auth.go

@@ -85,7 +85,7 @@ func VerifyPassword(hash, pw string) bool {
 
 // dummyHash is verified against when the user does not exist, so a login
 // for an unknown name takes as long as one for a known name.
-var dummyHash, _ = HashPassword("wgx-timing-equaliser-password")
+var dummyHash, _ = HashPassword("ihasvpn-timing-equaliser-password")
 
 // EqualiseTiming burns the cost of one hash verification.
 func EqualiseTiming() { VerifyPassword(dummyHash, "not-the-password") }