Redesign app icon: tilted dumbbell on purple gradient

Replace the teal circular mark with a full-bleed purple gradient and a
larger tilted dumbbell, applied across all iOS sizes and the Watch icon.
Icons are alpha-free RGB for App Store compliance.

CHANGELOG.md

@@ -4,6 +4,8 @@ All notable changes to this project are documented here.
 
 ## June 2026
 
+- New app icon: a tilted dumbbell on a purple gradient, full-bleed across iOS and
+  Watch (replaces the teal circular mark).
 - **2.0** — Re-platformed persistence onto an iCloud Drive document architecture:
   JSON files in iCloud Drive are now the sole source of truth, with a rebuildable
   SwiftData cache populated by an `NSMetadataQuery` observer. Removed