polish(web): favicon, Punktfunk casing, display-save toast, idle stop button

- Favicon: the brand-mark lens SVG at /favicon.svg, linked from the root head.
- Branding: user-facing "punktfunk" → "Punktfunk" (tab title, app_name,
  pairing prose, the logo aria-labels/SVG titles) — protocol identifiers
  (punktfunk/1, _punktfunk._udp, binary/module names) stay lowercase.
- The display policy auto-saves on every preset pick / field edit with no
  signal; mount @unom/ui's sonner Toaster in the root and fire a success toast
  from the save funnel's onSuccess (new i18n key, en + de). First toast usage
  in the console — errors stay inline as before.
- The stop-session button rendered destructive-red even with nothing to stop:
  variant switches to the muted `secondary` while no session is active.

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

web/public/favicon.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1000 1000">
+  <!-- Punktfunk brand mark: two overlapping circles forming a lens (see src/components/brand-mark.tsx). -->
+  <path d="M403.037,791.672c107.586,0 194.41,-86.824 194.41,-194.41c0,-107.586 -86.824,-194.41 -194.41,-194.41c-107.586,0 -194.41,86.824 -194.41,194.41c0,107.586 86.824,194.41 194.41,194.41Z" fill="#a79ff8"/>
+  <path d="M735.276,540.321c76.075,-76.075 76.075,-198.862 0,-274.937c-76.075,-76.075 -198.862,-76.075 -274.937,0c-76.075,76.075 -76.075,198.862 0,274.937c76.075,76.075 198.862,76.075 274.937,0Z" fill="#6c5bf3"/>
+  <path d="M647.84,590.737c-64.853,17.403 -136.871,0.597 -187.885,-50.416c-51.013,-51.013 -67.819,-123.032 -50.416,-187.885c64.853,-17.403 136.871,-0.597 187.885,50.416c51.013,51.013 67.819,123.032 50.416,187.885Z" fill="#d2c9fb"/>
+</svg>