v0.20.0: thinking blocks, chat windows overhaul, scroll fix

Backend:
- Add isThinking flag to agentEvent for text before tool calls
- Separate thinking from response text in runChatTurn and continue.go
- Persist thinking in a dedicated field in message content

Frontend:
- Add thinking field to MessageContent, ChatMessage, ChatTextEvent types
- Create ThinkingBlock.svelte — collapsible block with brain icon
- SSE handler moves text_delta content to thinking on isThinking flag
- Render thinking block between tools and response in ChatThread
- Fix chat window scroll reset on focus change (stable windowKeys order)
- Remove redundant #key id wrapper in WindowLayer
- Enlarge sidebar rail (24→32 default, 40→60 max)
- Remove glyph from sidebar, square graph at top
- Replace AgentTrace/ToolCallCard/UnifiedTimeline with TurnTrace/ToolLine

web/src/lib/stores/windows.ts

@@ -51,6 +51,22 @@ export const dk = createDesktop(wm, {
 })
 export const wmState = wmStore(wm)
 
+// Stable insertion-order window IDs — unlike $wmState.order (which reorders on
+// focus/raise), this only changes when a window is opened or closed. Used by
+// WindowLayer's {#each} so the DOM order stays stable; wmkit handles visual
+// stacking via z-index in syncAll(). Without this, every focus change moves
+// <section> elements in the DOM, which resets scroll positions of scrollable
+// children in Chrome.
+let _lastKeys: string[] = []
+export const windowKeys = derived(wmState, ($s) => {
+  const keys = Object.keys($s.windows)
+  if (keys.length === _lastKeys.length && keys.every((k, i) => k === _lastKeys[i])) {
+    return _lastKeys
+  }
+  _lastKeys = keys
+  return keys
+})
+
 // The session id backing whichever task/chat window currently has focus, or
 // null when no task window is focused (Tasks app, an entity window, or
 // nothing at all). The desktop mascot's stimuli (stimuli.ts) key off this so