feat(desktop): let every chat surface float its composer

popoutAllowed was hardcoded false for session tiles, so only the primary
thread could pop out — a tab or a split had no way to undock its composer.
Secondary windows stay docked; that gate was the load-bearing one.

apps/desktop/src/app/chat/composer/scope.tsx

@@ -23,8 +23,6 @@ export interface ComposerScope {
   /** This scope's "turn parked on user input" edge — gates Esc-to-stop. */
   $awaitingInput: ReadableAtom<boolean>
   attachments: ComposerAttachmentScope
-  /** Only the main scope may pop out (the floating composer is a singleton). */
-  popoutAllowed: boolean
   /** This scope's transcript. Read it imperatively (input-history browse) to
    *  keep streaming out of the composer's renders; subscribe only off-render
    *  (auto-speak) where the reply edge is the whole point. */
@@ -37,7 +35,6 @@ export const MAIN_COMPOSER_SCOPE: ComposerScope = {
   $awaitingInput: $activeSessionAwaitingInput,
   $messages,
   attachments: mainComposerScope,
-  popoutAllowed: true,
   target: 'main'
 }
 

apps/desktop/src/app/chat/session-tile.tsx

@@ -128,7 +128,6 @@ function TileChat({
       $awaitingInput: sessionAwaitingInput(runtimeId),
       $messages: view.$messages,
       attachments,
-      popoutAllowed: false,
       target: `tile:${storedSessionId}`
     }),
     [attachments, runtimeId, storedSessionId, view.$messages]