fix(streaming): strip Gemma 4 thinking token delimiter in all paths — closes #607

Fixes <|turn|>thinking delimiter (was wrong as <|turn>thinking) in api/streaming.py, static/messages.js, and static/ui.js. Adds 13 regression tests. Independent review by @nesquena.

static/messages.js

@@ -144,7 +144,8 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
   // Thinking tag patterns for streaming display
   const _thinkPairs=[
     {open:'<think>',close:'</think>'},
-    {open:'<|channel>thought\n',close:'<channel|>'}
+    {open:'<|channel>thought\n',close:'<channel|>'},
+    {open:'<|turn|>thinking\n',close:'<turn|>'}  // Gemma 4
   ];
 
   function _isActiveSession(){