fix: live transcription partials update in place instead of stacking rows

- lk.transcription handler now uses addPartial() (in-place update)
  instead of addMessage() (new row per word)
- addMessage() clears stale partials before rendering the final

web/app.js

@@ -264,6 +264,7 @@ function renderMessage(msg) {
 }
 
 function addMessage(role, text) {
+    clearPartials();
     const msg = { role, text, ts: Date.now() };
     history.push(msg);
     persistHistory();
@@ -335,7 +336,8 @@ function registerTranscriptionStream(rm) {
                     if (pub) { role = isAgent(p) ? "agent" : "user"; break; }
                 }
             }
-            addMessage(role, text.trim());
+            // Use partial rendering: updates the same row in place until final.
+            addPartial(role, text.trim());
         });
     } catch (e) {
         // Older SDKs may not support registerTextStreamHandler — the data