Make the app installable as a PWA

Adds a web manifest, icons (standard 192/512, a full-bleed maskable
512 with the glyph kept inside the safe zone, and an iOS touch icon),
theme-color/apple meta tags, and a service worker, so "Install app" /
"Add to Home Screen" gives it its own icon and a standalone window.

The service worker is a deliberate no-cache pass-through: it registers
a fetch listener (so browsers treat the app as installable) but never
calls respondWith(), so every request goes to the network exactly as
without a worker. A caching worker would keep serving an old JS bundle
after each rebuild — the same stale-build confusion that already cost
time on the settings layout fix — and this app is a live view of
authenticated data with no useful offline mode. Registered in
production builds only.

Icons are generated procedurally (server-rack glyph on the sidebar's
dark colour) and encoded as real PNGs; visually checked, including
that the maskable variant keeps the glyph inside the safe zone.

Verified in a real Chromium (headless Edge) over the DevTools protocol
against the built bundle served with the same express.static setup as
production: the DevTools installability audit reported no errors, the
manifest parsed with no errors, the worker registered at scope "/",
activated and took control, and a navigation through the worker
returned the app page normally. The manifest is served as
application/manifest+json and sw.js as JavaScript. The in-app preview
pane silently blocks service-worker script fetches, so it could not
be used for this — hence the real browser. Real dev database mtime
untouched throughout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/index.html

@@ -4,6 +4,13 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Homelab Manager</title>
+    <link rel="manifest" href="/manifest.webmanifest" />
+    <link rel="icon" type="image/png" href="/icon-192.png" />
+    <link rel="apple-touch-icon" href="/apple-touch-icon.png" />
+    <meta name="theme-color" content="#182433" />
+    <meta name="mobile-web-app-capable" content="yes" />
+    <meta name="apple-mobile-web-app-capable" content="yes" />
+    <meta name="apple-mobile-web-app-title" content="Homelab" />
     <script>
       (function () {
         try {