feat(web): new task opens straight into chat, context rail waits for content, frosted windows

New Task now opens directly as an empty ChatThread (NewTaskChat) instead of
a separate compose screen, sized like a real task window. The Scope/Activity
context rail in a task window no longer renders until there's actually
something to show (touched entities, activity, or an open question),
avoiding an empty-placeholder sidebar on every new task. Also fixes the
chat input defaulting to several lines tall on window open, centers the
empty-chat greeting vertically, and gives floating windows the same
frosted-glass look as the desktop's task launcher card.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/lib/components/SessionChatWindow.svelte

@@ -7,6 +7,7 @@
   import { Pane, Splitpanes } from 'svelte-splitpanes'
   import { chatFor, loadSessionChat, sendSessionMessage, cancelSessionStream, stopSessionPolling, dismissError, chatErrors } from '$lib/stores/chat'
   import { activityLogFor } from '$lib/stores/activity'
+  import { workspaceFor, startSessionWorkspace } from '$lib/stores/workspace'
   import ChatThread from '$lib/components/ChatThread.svelte'
   import TaskContextPanel from '$lib/components/TaskContextPanel.svelte'
 
@@ -26,14 +27,46 @@
   const chatNotFound = chat.notFound
   // eslint-disable-next-line svelte/valid-compile
   const sessionActivityLog = activityLogFor(sessionId)
+  // Started here (rather than left to TaskContextPanel's own onMount) so the
+  // workspace is already tracking touched entities/plan/questions before the
+  // context rail ever mounts — it needs that live even while the rail stays
+  // hidden (see hasContext below).
+  // eslint-disable-next-line svelte/valid-compile
+  const workspace = workspaceFor(sessionId)
+  // eslint-disable-next-line svelte/valid-compile
+  const touchedEntities = workspace.touched
+  // eslint-disable-next-line svelte/valid-compile
+  const openQuestion = workspace.openQuestion
   let loading = $state(true)
 
+  // The context rail (Scope/Activity) is only worth its screen space once
+  // there's something in it — a brand-new task otherwise opens to an empty
+  // "entities appear here" placeholder next to an equally empty activity
+  // list. Show it the moment any of the three has real content, and keep it
+  // shown from then on (no flicker back to hidden if e.g. touched entities
+  // later expire).
+  let hasContext = $state(false)
+  $effect(() => {
+    if (!hasContext && ($sessionActivityLog.length > 0 || $touchedEntities.length > 0 || $openQuestion !== null)) {
+      hasContext = true
+    }
+  })
+
+  // startSessionWorkspace's cleanup is registered via onDestroy below rather
+  // than returned from this callback — onMount ignores a returned function
+  // once the callback is async (its return value is a Promise, not the
+  // cleanup itself).
+  const stopWorkspace = startSessionWorkspace(sessionId)
+
   onMount(async () => {
     await loadSessionChat(sessionId)
     loading = false
   })
 
-  onDestroy(() => stopSessionPolling(sessionId))
+  onDestroy(() => {
+    stopSessionPolling(sessionId)
+    stopWorkspace()
+  })
 
   // Resizable right rail — sized smaller by default since task windows open
   // narrower than the full page.
@@ -48,7 +81,7 @@
       <p class="text-sm text-muted-foreground">Task not found.</p>
       <p class="text-xs text-muted-foreground/70">It may have been deleted.</p>
     </div>
-  {:else}
+  {:else if hasContext}
     <Splitpanes theme="oikos-theme" dblClickSplitter={false}>
       <Pane>
         <ChatThread
@@ -68,5 +101,18 @@
         <TaskContextPanel {sessionId} />
       </Pane>
     </Splitpanes>
+  {:else}
+    <ChatThread
+      messages={$chatMessages}
+      streaming={$chatStreaming}
+      connectionState={$chatConnectionState}
+      error={$chatError}
+      chatErrors={$chatErrors}
+      activityLog={sessionActivityLog}
+      onSend={(text) => sendSessionMessage(sessionId, text)}
+      onCancel={() => cancelSessionStream(sessionId)}
+      onReconnect={() => loadSessionChat(sessionId)}
+      onDismissError={dismissError}
+    />
   {/if}
 </div>