fix: stop transcript leakage and repetition loops in Generic chat template

Backend (inference-server/compat.rs):
- PreparedPrompt now carries extra_stop_sequences injected by template
- Generic template injects User:/System:/Assistant:/Human: stop sequences
  to prevent the model from continuing the transcript after its reply
- Increased repeat_penalty 1.15→1.3 and repeat_last_n 256→512 to
  break repetition loops more aggressively

Backend (inference-server/routes.rs):
- Merge template extra_stop_sequences into sampler before inference

Frontend (chat.ts):
- Added 'System:' and 'Human:' to CHAT_TRANSCRIPT_STOP_SEQUENCES
- Extended TRANSCRIPT_LEAKAGE_PATTERN to include 'Human:'

Frontend (ChatView.tsx):
- buildApiMessages now strips leaked transcript from assistant messages
  before sending history back to the model
- Reduced history window to 20 messages to limit context pollution

crates/inference-server/src/routes.rs

@@ -167,10 +167,21 @@ async fn chat_completions_handler(
         );
     }
 
-    let sampler = build_sampler(&req);
+    let mut sampler = build_sampler(&req);
     let prepared = prepare_prompt(&req, &state.server_config);
     tracing::debug!(prompt_len = prepared.prompt.len(), "chat_completions: prompt prepared");
 
+    // Merge template-injected stop sequences with request-provided ones
+    if !prepared.extra_stop_sequences.is_empty() {
+        let mut stops = sampler.stop_sequences.clone();
+        for s in &prepared.extra_stop_sequences {
+            if !stops.contains(s) {
+                stops.push(s.clone());
+            }
+        }
+        sampler.stop_sequences = stops;
+    }
+
     if req.stream {
         tracing::info!("chat_completions: starting stream response");
         stream_response(