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/stores/windows.ts

@@ -22,3 +22,20 @@ export function openEntityWindow(slug: string | null): void {
   }
   wm.open({ id: slug, title: slug })
 }
+
+// Same dedupe/restore/focus pattern as openEntityWindow, for a task/session's
+// chat window. Id is namespaced `session:<id>` — distinct from entity window
+// ids (always a bare `type:identifier` slug, and task ENTITIES already use
+// `task:<uuid>` as their own slug) so a task's chat window and its entity
+// detail window never collide over the same wmkit id. See
+// EntityDesktop.svelte for the id -> content-component branch.
+export function openTaskWindow(sessionId: string | null, title: string): void {
+  if (!sessionId) return
+  const id = `session:${sessionId}`
+  if (wm.get(id)) {
+    wm.restore(id)
+    wm.focus(id)
+    return
+  }
+  wm.open({ id, title, width: 900, height: 640 })
+}