spotify: resilient fetching, progress bars, venv launcher

- fetch() uses a retrying session (connection resets, 5xx, 429 Retry-After)
  with backoff, so a transient drop mid-fetch no longer aborts the run.
- tqdm progress bars on all paginated fetches (driven by paging `total`).
- Flow.fetch_spotify(): drop stray `self` from the @staticmethod.
- gui.sh runs the venv's streamlit from the script dir; add requirements.txt.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Flow.py

@@ -26,7 +26,7 @@ class Flow:
     itunes_library : Library = None
 
     @staticmethod
-    def fetch_spotify(self):
+    def fetch_spotify():
         fetch_all(client_id, client_secret, work_dir)
 
     @staticmethod