Implement profile deletion with undo functionality

- Add delete_profile() to profiles.rs - moves profile to undo stack
- Add undo_history field and undo_delete() method to App struct
- Add cli_delete_profile() and cli_undo_delete() CLI commands
- Add delete-profile <npub> and undo-delete to USAGE
- Update CHECKPOINT-encryption.md checkpoint

Verification: cargo test (91 passed), clippy clean, fmt clean, release build successful

frontend/assets/icons/hummingbird-key.svg

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="64" height="64" viewBox="0 0 64 64" version="1.1" xmlns="http://www.w3.org/2000/svg">
+  <!-- Main hummingbird body forming the key bow -->
+  <path d="M14 46 C22 20, 40 20, 48 40 C52 52, 40 58, 30 52 Z" fill="#FF6B6E" opacity="0.9"/>
+  <!-- Hummingbird wings - left and right -->
+  <path d="M20 38 C18 30, 22 25, 28 28 L32 32 L36 28 C42 25, 46 30, 44 38 Z" fill="none" stroke="#FF6B6E" stroke-width="2"/>
+  <path d="M50 38 C48 30, 54 25, 58 28 L62 32 L66 28 C70 25, 74 30, 72 38 Z" fill="none" stroke="#FF6B6E" stroke-width="2"/>
+  <!-- Hummingbird head - becomes key's keyring area -->
+  <circle cx="54" cy="32" r="6" fill="#FF6B6E"/>
+  <!-- Key tip / beak - the most prominent point -->
+  <path d="M58 32 L62 24 L58 16 Z" fill="#FF6B6E"/>
+  <!-- Key stem/shank from beak downward -->
+  <path d="M58 16 L58 10 L62 6 L68 10 L68 16 Z" fill="#FF6B6E"/>
+  <!-- Decorative key teeth along the stem -->
+  <path d="M62 8 L66 12 L62 16" fill="#FF6B6E"/>
+  <path d="M62 10 L66 14 L62 18" fill="#FF6B6E"/>
+</svg>