Add saved channel keys feature for Web UI users

- Database: Add user_channel_keys table with CASCADE delete
- Auth: Add CRUD functions for channel key management (10 keys/user limit)
- Routes: Add key save/delete/rename endpoints and JSON API
- Account page: Add saved keys section with add/rename/delete UI
- Encode/Decode: Add saved keys to channel key dropdown (optgroup)
- About page: Add Channel Key QR generator for sharing keys
- Track last_used_at when saved keys are used

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontends/web/static/js/stegasoo.js

@@ -819,6 +819,14 @@ const Stegasoo = {
             // Set the select value to the actual key for form submission
             select.value = keyInput.value;
         }
+
+        // Track saved key usage (fire-and-forget)
+        const selectedOption = select?.selectedOptions?.[0];
+        const keyId = selectedOption?.dataset?.keyId;
+        if (keyId) {
+            fetch(`/api/channel/keys/${keyId}/use`, { method: 'POST' }).catch(() => {});
+        }
+
         return true;
     },