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>

main.py

@@ -17,17 +17,17 @@ def test1():
 def main():
     flow = Flow.Flow()
 
-    # flow.fetch_spotify()
+    flow.fetch_spotify()
     # flow.fetch_local()
 
     flow.parse_spotify_library()
-    flow.parse_itunes_library()
-    flow.parse_local_library()
+    #flow.parse_itunes_library()
+    #flow.parse_local_library()
 
-    flow.load_libraries()
+    #flow.load_libraries()
 
-    flow.map_local_to_spotify()
-    flow.save_libraries()
+    #flow.map_local_to_spotify()
+    #flow.save_libraries()
 
     # flow.log_stats()