swarm-ui: make it installable as a PWA

Closes #4282. mara: "scope looks good" — approving the plan posted
there (manifest + icons + minimal shell-only service worker + iOS meta
tags) and both explicit questions (network-first-with-offline-fallback,
never cache /api/*).

docs/web-ui/design-guide.md's "Layout & viewport" section already
asserted swarm-ui is installable as a PWA — this is what actually backs
it.

- manifest.webmanifest: name/icons/start_url/standalone display, theme
  #cba6f7 / background #1e1e2e matching the mocha --purple/--bg values.
- sw.js: plain JS, not TypeScript — the DOM lib swarm-ui's own tsconfig
  uses and the WebWorker lib a service worker's globals need are
  mutually exclusive in one tsc program, not worth a second tsconfig for
  a self-contained ~100-line file. Scoped to the app shell only, never
  touches /api/* at all, network-first with offline-fallback-to-cache
  (not cache-first) since main.js/main.css are unhashed filenames and a
  cache-first SW would risk wedging an operator on stale JS after a
  deploy.
- index.html: manifest link, theme-color meta, iOS
  apple-mobile-web-app-* tags (Safari ignores the manifest spec).
- main.tsx: feature-detected SW registration.
- branding/hyperhive-maskable.svg: hyperhive.svg's own artwork already
  fills nearly its whole canvas, so a maskable icon needs a padded
  variant or an OS mask crops the outer ring/corner brackets — embeds
  the original via a scaled <image> ref rather than duplicating markup.
- nix/packages/swarm-ui.nix: rasterizes icon-192/512/512-maskable.png
  from the branding SVGs at build time via librsvg, rather than
  checking in static PNGs.

Verified for real: typecheck+build green, and a real headless-chromium
tab driven over CDP confirms the service worker registers and becomes
the active controller, and a simulated-offline reload still serves the
full cached shell rather than a browser error page. nix build .#swarm-ui
also verified green, including the rasterized icon output.

frontend/packages/swarm-ui/src/index.html

@@ -5,6 +5,19 @@
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <title>hyperhive swarm</title>
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
+    <!-- PWA installability — design-guide.md's "Layout & viewport"
+         section already asserted swarm-ui is installable; this is what
+         actually backs it. `manifest.webmanifest` + icons are
+         nix-packaged (nix/packages/swarm-ui.nix), same as favicon.svg
+         above — not part of the plain npm build output. -->
+    <link rel="manifest" href="/manifest.webmanifest" />
+    <meta name="theme-color" content="#cba6f7" />
+    <!-- Safari ignores the manifest spec entirely — these are its own
+         "Add to Home Screen" affordances, same icon/name the manifest
+         already declares for every other browser. -->
+    <meta name="apple-mobile-web-app-capable" content="yes" />
+    <meta name="apple-mobile-web-app-title" content="swarm" />
+    <link rel="apple-touch-icon" href="/icon-192.png" />
     <link rel="stylesheet" href="/static/colors.css" />
     <link rel="stylesheet" href="/static/theme.css" />
     <link rel="stylesheet" href="/static/swarm-ui.css" />