fix: clean contextualization artifacts and ensure passages start properly

- Remove word-colon patterns like 'mas:' from AI responses
- Always start passages with first word of complete sentence
- Find capital letter after punctuation for proper sentence starts
- Fallback to first capital letter if no sentence break found

src/aiService.js

@@ -74,10 +74,10 @@ class OpenRouterService {
       
       let content = data.choices[0].message.content.trim();
       
-      // Clean up AI response artifacts
+      // Clean up AI response artifacts  
       content = content
         .replace(/^\s*["']|["']\s*$/g, '')  // Remove leading/trailing quotes
-        .replace(/^\s*:+\s*/, '')           // Remove leading colons
+        .replace(/^\s*[:;]+\s*/, '')        // Remove leading colons and semicolons
         .replace(/\*+/g, '')                // Remove asterisks (markdown bold/italic)
         .replace(/_+/g, '')                 // Remove underscores (markdown)
         .replace(/#+\s*/g, '')              // Remove hash symbols (markdown headers)
@@ -212,7 +212,8 @@ Passage: "${passage}"`
       // Clean up AI response artifacts
       content = content
         .replace(/^\s*["']|["']\s*$/g, '')  // Remove leading/trailing quotes
-        .replace(/^\s*:+\s*/, '')           // Remove leading colons
+        .replace(/^\s*[:;]+\s*/, '')        // Remove leading colons and semicolons
+        .replace(/^\s*[a-z]+:\s*/i, '')     // Remove any word followed by colon (like "mas:")
         .replace(/\*+/g, '')                // Remove asterisks (markdown bold/italic)
         .replace(/_+/g, '')                 // Remove underscores (markdown)
         .replace(/#+\s*/g, '')              // Remove hash symbols (markdown headers)

src/clozeGameEngine.js

@@ -76,10 +76,17 @@ class ClozeGame {
     // Extract longer initial passage for better sentence completion
     let passage = text.substring(startIndex, startIndex + 1000);
     
-    // Clean up start - find first complete sentence
-    const firstSentenceEnd = passage.search(/[.!?]\s+[A-Z]/);
-    if (firstSentenceEnd > 0 && firstSentenceEnd < 200) {
-      passage = passage.substring(firstSentenceEnd + 2);
+    // Clean up start - find first complete sentence that starts with capital letter
+    const firstSentenceMatch = passage.match(/[.!?]\s+([A-Z][^.!?]*)/);
+    if (firstSentenceMatch && firstSentenceMatch.index < 200) {
+      // Start from the capital letter after punctuation
+      passage = passage.substring(firstSentenceMatch.index + firstSentenceMatch[0].length - firstSentenceMatch[1].length);
+    } else {
+      // If no good sentence break found, find first capital letter
+      const firstCapitalMatch = passage.match(/[A-Z][^.!?]*/);
+      if (firstCapitalMatch) {
+        passage = passage.substring(firstCapitalMatch.index);
+      }
     }
     
     // Clean up end - ensure we end at a complete sentence