feat: add Thai language support (100% Thai UI)

- Add locales/th.json with 629 translated keys
- Add Thai to languages.json with llmInstruction for report generation
- Backend auto-loads new locale files from locales/ directory

locales/languages.json

@@ -26,5 +26,9 @@
   "de": {
     "label": "Deutsch",
     "llmInstruction": "Bitte antworten Sie auf Deutsch."
+  },
+  "th": {
+    "label": "ไทย",
+    "llmInstruction": "กรุณาตอบเป็นภาษาไทย"
   }
 }