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/agent.go

@@ -181,6 +181,11 @@ type agentEvent struct {
 	Data      any    `json:"data,omitempty"`
 	SessionID string `json:"session_id,omitempty"`
 	Iteration int    `json:"iteration,omitempty"`
+	// IsThinking marks text/text_delta events that carry the model's internal
+	// reasoning (text produced before tool calls in the same iteration), as
+	// distinct from the final response text. The frontend renders these as
+	// collapsible thinking blocks separated from the response.
+	IsThinking bool `json:"is_thinking,omitempty"`
 }
 
 func (a *agent) chat(ctx context.Context, sessionID, message string, emit func(agentEvent)) {
@@ -465,7 +470,7 @@ func (a *agent) chatWith(ctx context.Context, sessionID, message, systemInject s
 		// led to each step. Emitting it lets the persist layer accumulate
 		// per-iteration reasoning into the row's text field.
 		if strings.TrimSpace(msg.Content) != "" {
-			emit(agentEvent{Type: "text", Data: msg.Content, SessionID: sessionID})
+			emit(agentEvent{Type: "text", Data: msg.Content, SessionID: sessionID, IsThinking: true})
 		}
 
 		slog.Info("nomos: tool calls", "count", len(msg.ToolCalls), "iter", i+1, "correlation", correlationID)