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/activity.ts

@@ -1,6 +1,7 @@
-import { derived } from 'svelte/store'
-import { messages, type ToolCallResult } from './chat'
-import { planSteps, currentTask } from './workspace'
+import { derived, type Readable } from 'svelte/store'
+import { messages, chatFor, type ChatMessage, type ToolCallResult } from './chat'
+import { planSteps, currentTask, workspaceFor, taskFor } from './workspace'
+import type { PlanStep, Session } from '$lib/api'
 
 export { type ToolCallResult }
 
@@ -39,7 +40,10 @@ function stringifyResult(result: unknown): string {
   return s.length > DETAIL_MAX ? `${s.slice(0, DETAIL_MAX)}\n… truncated` : s
 }
 
-export const activityLog = derived([messages, planSteps, currentTask], ([$msgs, $steps, $task]) => {
+// Pure derivation, parameterized so it can back both the global "current
+// session" activityLog below and a per-session activityLogFor(sessionId) for
+// a floating task window.
+function computeActivityLog($msgs: ChatMessage[], $steps: PlanStep[], $task: Session | null): ActivityEntry[] {
   const entries: ActivityEntry[] = []
   const now = Date.now()
 
@@ -168,7 +172,18 @@ export const activityLog = derived([messages, planSteps, currentTask], ([$msgs,
   entries.sort((a, b) => a.timestamp - b.timestamp)
 
   return entries
-})
+}
+
+export const activityLog = derived([messages, planSteps, currentTask], ([$msgs, $steps, $task]) =>
+  computeActivityLog($msgs, $steps, $task)
+)
+
+export function activityLogFor(sessionId: string): Readable<ActivityEntry[]> {
+  const chat = chatFor(sessionId)
+  const ws = workspaceFor(sessionId)
+  const task = taskFor(sessionId)
+  return derived([chat.messages, ws.planSteps, task], ([$msgs, $steps, $task]) => computeActivityLog($msgs, $steps, $task))
+}
 
 function toolActivityLabel(t: ToolCallResult): string {
   const args = t.args ?? {}