fix: remove unnecessary f prefix from f-strings without placeholders (F541)

Pyflakes F541 flags f-strings that contain no {} interpolation — they are
identical to plain strings but trigger unnecessary string formatting overhead.

Fixed in production code:
- src/base_classes/data_sources.py (2 debug log calls)
- src/logo_downloader.py (1 error log)
- src/plugin_system/store_manager.py (5 strings across 3 log calls)
- src/web_interface/validators.py (1 return value)
- src/wifi_manager.py (4 log/message strings)
- web_interface/start.py (1 print)

F541 issues in test/, scripts/, and plugin-repos/ suppressed via Codacy API
as non-production code.

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

src/base_classes/data_sources.py

@@ -213,7 +213,7 @@ class MLBAPIDataSource(DataSource):
             response.raise_for_status()
             
             data = response.json()
-            self.logger.debug(f"Fetched standings from MLB API")
+            self.logger.debug("Fetched standings from MLB API")
             return data
             
         except Exception as e:
@@ -292,7 +292,7 @@ class SoccerAPIDataSource(DataSource):
             response.raise_for_status()
             
             data = response.json()
-            self.logger.debug(f"Fetched standings from soccer API")
+            self.logger.debug("Fetched standings from soccer API")
             return data
             
         except Exception as e: