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>

test/e2e/app.e2e.ts

@@ -886,6 +886,8 @@ try {
   // Custom icon from the tile's context menu: square-cropped and shown instead of initials
   const iconFile = path.join(userData, 'icon.png');
   writeFileSync(iconFile, testPng(300, 200));
+  // With hints off, the title of whatever the pointer rests on is set aside; move off the tile first
+  await page.mouse.move(700, 400);
   await page.locator('.rail .tile[title^="Second"]').click({ button: 'right' });
   const chooser = page.waitForEvent('filechooser');
   await page.getByRole('menuitem', { name: 'Change icon...' }).click();