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 {

docs/systems/voice.md

@@ -236,6 +236,14 @@ ScreenShareConfig {
 
 ---
 
+## Voice Fullscreen
+
+The fullscreen toggle in `VoiceControlBar` flips the `voiceFullscreen` flag in `uiStore`; an effect in `MainContent.tsx` calls `voiceContainerRef.current.requestFullscreen()` (and exits via `document.exitFullscreen()` when the flag clears). A second effect listens to `fullscreenchange` and reflects the actual `document.fullscreenElement` back into the store, so pressing Esc or system-level fullscreen-exit keeps state in sync. `voiceChatOpen && !voiceFullscreen` hides the side chat panel while fullscreen is active.
+
+**Overlay portals:** While fullscreen is active the browser's Fullscreen API renders only descendants of `voiceContainerRef`. Every overlay reachable during a call (context menus on `StreamTile`/`VoiceUser`/`VoiceChannel`, tooltips on the control bar, `ConnectionInfoPopover`, `ScreenShareSettingsPopover`, `ConfirmDialog` invoked from voice context-menu actions, and `ScreenSharePicker`) portals through `usePortalContainer()` so it lands inside the fullscreen element. Adding new overlays that can be opened from inside the call must follow the same contract — see `docs/systems/design-system.md` Surface Material Tiers.
+
+---
+
 ## Audio Processing
 
 | Feature | Default | User Control | Notes |