fix(web): portal overlays into fullscreenElement so they render in voice fullscreen

requestFullscreen() on the voice container puts only its descendants in the
browser's top layer; overlays portaled to document.body were rendered outside
that layer and stayed invisible — most visibly the right-click context menu on
stream tiles and voice user panels.

Add usePortalContainer() hook returning document.fullscreenElement ?? document.body
and re-rendering on fullscreenchange. Migrate every overlay reachable during a
call: ContextMenuRenderer (desktop, submenu, mobile sheet), Tooltip,
ConfirmDialog, ConnectionInfoPopover, ScreenShareSettingsPopover, and
ScreenSharePicker (which previously rendered inline at App root).

docs/systems/design-system.md

@@ -63,6 +63,8 @@ Font: DM Sans (primary) with system fallbacks
 
 **Modal backdrops:** `bg-black/50` — light enough for glass blur to show through.
 
+**Portal target — `usePortalContainer()`:** Every overlay (context menu, tooltip, popover, modal, screen-share picker) MUST portal through `usePortalContainer()` (`packages/web/src/hooks/usePortalContainer.ts`) instead of hard-coding `document.body`. The hook returns `document.fullscreenElement ?? document.body` and re-renders subscribers on `fullscreenchange`. Without this, anything portaled while an element (e.g. the voice container in fullscreen mode) is in the browser's Fullscreen API top-layer is rendered outside that layer and is invisible. Components mounted at App root that render with `fixed inset-0` (not just portals) must also portal through this hook for the same reason.
+
 ### Glass Material Properties
 ```css
 .glass {