Rename iPodderX to iPX (#56)

The app, the repository (rays/ipx), the image, the compose service and
container, and the data folders on Tower take the new name. What stays:
the 2004 iPodderX and ipodderx-core, which are history; the Postgres
database and login, and ipodderx.sdf1.net with its Access and Authentik
apps, which would each need moving outside this repo; and the first-start
password, as ipx is shorter than the eight characters a password needs.

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

web/login.html

@@ -3,7 +3,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <meta name="color-scheme" content="dark light">
-<title>Sign in — iPodderX</title>
+<title>Sign in — iPX</title>
 <link rel="icon" type="image/png" sizes="128x128" href="/favicon.png">
 <link rel="apple-touch-icon" href="/apple-touch-icon.png">
 <style>
@@ -74,7 +74,7 @@ button:focus-visible{outline:2px solid var(--accent);outline-offset:2px}
 </style>
 
 <form id="f">
-  <div class="brand"><img src="/logo.svg" alt=""><h1>iPodderX</h1></div>
+  <div class="brand"><img src="/logo.svg" alt=""><h1>iPX</h1></div>
   <label for="name">Name</label>
   <input id="name" name="name" autocomplete="username" autofocus required>
   <label for="pw">Password</label>