feat(gui): aggiunge interfaccia desktop PySide6 con tema light/dark

- chat con Markdown e formule KaTeX (via QWebEngineView + CDN)
- top bar con selezione collezione ChromaDB e bottone ☀️/🌙
- input multiriga con Enter per inviare e Shift+Enter per andare a capo
- animazione thinking durante la generazione
- fonti dei chunk mostrate sotto ogni risposta

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui/worker.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+from PySide6.QtCore import QThread, Signal
+import chromadb
+
+
+class RagWorker(QThread):
+    finished = Signal(str, list)   # response, chunks
+    error    = Signal(str)
+
+    def __init__(self, question: str, collection: chromadb.Collection, parent=None):
+        super().__init__(parent)
+        self.question   = question
+        self.collection = collection
+
+    def run(self) -> None:
+        try:
+            from rag.rag import retrieve, build_prompt, call_ollama
+            chunks        = retrieve(self.collection, self.question)
+            system, prompt = build_prompt(self.question, chunks)
+            response      = call_ollama(prompt, system=system)
+            self.finished.emit(response, chunks)
+        except Exception as exc:
+            self.error.emit(str(exc))