feat(chat): redesign input bar, expand sidebar, add thinking toggle

- Restructure floating input: dominant textarea with compact bottom
  toolbar (model badge, thinking toggle, attach, send/stop).
- Model badge sizes to the current selection (not widest option) via
  a layered transparent select, with truncate-on-overflow fallback.
- Auto-expand the conversation sidebar on desktop and slide chat
  content right when open instead of overlaying.
- Add per-request thinking toggle (brain icon, default on, persisted
  in localStorage) sending chat_template_kwargs.enable_thinking.
- Always disable thinking for title summarization.
- Generate chat titles before the main response to keep the SSE
  stream from staying open past visible completion and to avoid
  busting the KV cache between turns.

backend/internal/client/client_test.go

@@ -32,7 +32,7 @@ func TestSendMessage(t *testing.T) {
 	_, err = client.SendMessage(ctx, []*store.Message{{
 		Role:    "user",
 		Content: "What is 2+2? Think step by step.",
-	}}, model, func(mc *MessageChunk) error {
+	}}, model, true, func(mc *MessageChunk) error {
 		if mc.Thinking != nil {
 			_, err := thinkingBuf.Write([]byte(*mc.Thinking))
 			return err
@@ -118,7 +118,7 @@ func TestSendMessageWithImage(t *testing.T) {
 		Role:    "user",
 		Content: "Describe this image in detail.",
 		Images:  []string{dataURL},
-	}}, model, func(mc *MessageChunk) error {
+	}}, model, true, func(mc *MessageChunk) error {
 		if mc.Message != nil {
 			outputBuf.WriteString(*mc.Message)
 		}