Screen sharing: start dialog, sound, video tiles and a meeting view

- Start dialog with source list, live preview, sound and quality choices
- Sound on Linux through PipeWire: one program or everything except this
  app, fed into a virtual microphone and kept linked as programs come and go;
  system loopback on Windows; stereo Opus
- A sharer's tile shows the picture; clicking it opens a large view with the
  other tiles as a strip. Own stream is previewed in the own tile
- Stream volume for viewers, watch and stop from the profile panel
- Unit tests for the PipeWire link planner; the proxy test no longer writes to
  the root channel other tests listen in

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

src/ui/Chat.svelte

@@ -3,7 +3,6 @@
   import ChatLog from './ChatLog.svelte';
   import Composer from './Composer.svelte';
   import VoiceStage from './VoiceStage.svelte';
-  import ShareView from './ShareView.svelte';
   import { ui } from '../lib/ui.svelte.ts';
   import { session } from '../lib/session.svelte.ts';
 
@@ -28,8 +27,7 @@
     <button class="icon-btn" onclick={onpanel} title="Members"><Icon name="users" /></button>
   </header>
 
-  {#if showStage && session.status === 'connected'}<VoiceStage />{/if}
-  {#if session.status === 'connected'}<ShareView />{/if}
+  {#if session.status === 'connected'}<VoiceStage always={showStage} />{/if}
 
   {#if session.status === 'idle' && session.disconnectInfo}
     {@const d = session.disconnectInfo}