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

cmd/nomos/main.go

@@ -180,7 +180,9 @@ func (a *agent) runChatTurn(pctx, ctx context.Context, sessionID, message string
 	// P3: accumulate per-iteration reasoning instead of overwriting with the
 	// final `text` event (see the original inline comment in handleChat).
 	var textParts []string
+	var thinkingParts []string
 	var finalText string
+	var finalThinking string
 
 	placeholder, _ := json.Marshal(map[string]any{"role": "assistant", "text": ""})
 	msgID, err := a.store.insertMessageReturningID(pctx, sessionID, "assistant", placeholder)
@@ -194,6 +196,7 @@ func (a *agent) runChatTurn(pctx, ctx context.Context, sessionID, message string
 		body, _ := json.Marshal(map[string]any{
 			"role":       "assistant",
 			"text":       finalText,
+			"thinking":   finalThinking,
 			"tool_calls": toolCalls,
 		})
 		a.store.updateMessage(pctx, msgID, body)
@@ -223,8 +226,13 @@ func (a *agent) runChatTurn(pctx, ctx context.Context, sessionID, message string
 		}
 		if ev.Type == "text" {
 			if t, ok := ev.Data.(string); ok && t != "" {
-				textParts = append(textParts, t)
-				finalText = strings.Join(textParts, "\n\n")
+				if ev.IsThinking {
+					thinkingParts = append(thinkingParts, t)
+					finalThinking = strings.Join(thinkingParts, "\n\n")
+				} else {
+					textParts = append(textParts, t)
+					finalText = strings.Join(textParts, "\n\n")
+				}
 				persist()
 			}
 		}