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/App.svelte

@@ -9,6 +9,7 @@
   import Learning from './pages/Learning.svelte'
   import Config from './pages/Config.svelte'
   import EntityDesktop from '$lib/components/EntityDesktop.svelte'
+  import MinimizedWindowsBar from '$lib/components/MinimizedWindowsBar.svelte'
   import { newChat } from '$lib/stores/chat'
   import { summary, subscribeContext, openSignalCount } from '$lib/stores/context'
   import { currentTask } from '$lib/stores/workspace'
@@ -235,6 +236,7 @@
         <Chat />
       {/if}
     </main>
+    <MinimizedWindowsBar />
   </Sidebar.Inset>
 </Sidebar.Provider>