feat(web): open tasks/sessions as floating windows with independent live chat

Clicking a task now opens it as a wmkit floating window (like entity
windows already do) instead of navigating away from wherever you were.
Several task windows can be open and actively streaming at once, each
fully independent — no "which one's on screen" guard needed, since
each window owns its own store bundle:

- chat.ts: chatFor(sessionId)/loadSessionChat/sendSessionMessage give
  each window its own messages/streaming/connectionState, alongside
  the existing singleton path the main Chat page still uses unchanged.
- workspace.ts: same split for plan/questions/touched/health-diffs
  (workspaceFor/startSessionWorkspace), each with its own live-event
  watermark since several windows can watch the same event stream.
- activity.ts: activityLogFor(sessionId) mirrors the global derivation.

SessionGraph.svelte, OperatorQuestion.svelte, and ActivityTimeline.svelte
were converted from store-importing to prop-driven (matching the new
ChatThread.svelte, extracted from Chat.svelte's transcript/input so both
the main page and task windows share one implementation instead of
duplicating markup/styling) so each can render either the global
"current session" or a specific window's session.

Also: minimized-window taskbar chips now cap at a max width with
middle-ellipsis truncation instead of growing unbounded, and the
window header's title/action-button row is fixed to genuinely match
heights (not just share a center point) for more robust alignment.

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

web/src/lib/components/SessionChatWindow.svelte

@@ -0,0 +1,91 @@
+<script lang="ts">
+  // Floating-window content for a task/session — the per-window counterpart
+  // to the main Chat page (thread + rail), fully self-contained per
+  // sessionId via chat.ts's chatFor()/loadSessionChat()/sendSessionMessage()
+  // and workspace.ts's workspaceFor()/startSessionWorkspace(), so several of
+  // these can be open (and independently live) at once without the "which
+  // one's on screen" guarding the main page's singleton stores need.
+  import { onDestroy, onMount } from 'svelte'
+  import { chatFor, loadSessionChat, sendSessionMessage, cancelSessionStream, stopSessionPolling, dismissError, chatErrors } from '$lib/stores/chat'
+  import ChatThread from '$lib/components/ChatThread.svelte'
+  import TaskContextPanel from '$lib/components/TaskContextPanel.svelte'
+
+  let { sessionId }: { sessionId: string } = $props()
+
+  // Svelte's `$store` auto-subscription only works on a plain identifier
+  // bound directly to a store, not a member expression — chatFor() returns
+  // an object of stores, so pull each one out into its own identifier here.
+  const chat = chatFor(sessionId)
+  const chatMessages = chat.messages
+  const chatStreaming = chat.streaming
+  const chatConnectionState = chat.connectionState
+  const chatError = chat.error
+  let loading = $state(true)
+
+  onMount(async () => {
+    await loadSessionChat(sessionId)
+    loading = false
+  })
+
+  onDestroy(() => stopSessionPolling(sessionId))
+
+  // Resizable right rail — same behavior as Chat.svelte's, sized smaller by
+  // default since task windows open narrower than the full page.
+  const RAIL_MIN = 220
+  const RAIL_MAX = 480
+  let railWidth = $state(260)
+  let resizing = $state(false)
+
+  function startResize(e: PointerEvent) {
+    e.preventDefault()
+    resizing = true
+    const startX = e.clientX
+    const startW = railWidth
+    function move(ev: PointerEvent) {
+      railWidth = Math.min(RAIL_MAX, Math.max(RAIL_MIN, startW + (startX - ev.clientX)))
+    }
+    function up() {
+      resizing = false
+      window.removeEventListener('pointermove', move)
+      window.removeEventListener('pointerup', up)
+    }
+    window.addEventListener('pointermove', move)
+    window.addEventListener('pointerup', up)
+  }
+</script>
+
+<div class="flex h-full min-h-0">
+  {#if loading}
+    <div class="flex flex-1 items-center justify-center text-xs text-muted-foreground">Loading…</div>
+  {:else}
+    <ChatThread
+      messages={$chatMessages}
+      streaming={$chatStreaming}
+      connectionState={$chatConnectionState}
+      error={$chatError}
+      chatErrors={$chatErrors}
+      onSend={(text) => sendSessionMessage(sessionId, text)}
+      onCancel={() => cancelSessionStream(sessionId)}
+      onReconnect={() => loadSessionChat(sessionId)}
+      onDismissError={dismissError}
+    />
+
+    <div class="flex shrink-0" style="width: {railWidth}px">
+      <button
+        type="button"
+        class="group/rz relative w-1.5 shrink-0 cursor-col-resize touch-none"
+        onpointerdown={startResize}
+        aria-label="Resize task panel"
+      >
+        <span
+          class="absolute inset-y-0 left-1/2 w-px -translate-x-1/2 transition-colors {resizing
+            ? 'bg-primary/60'
+            : 'bg-border group-hover/rz:bg-primary/50'}"
+        ></span>
+      </button>
+      <div class="flex min-w-0 flex-1 flex-col">
+        <TaskContextPanel {sessionId} />
+      </div>
+    </div>
+  {/if}
+</div>