feat: implement configuration management and enhance user registration flow

k-tv-frontend/app/(main)/docs/page.tsx

@@ -758,7 +758,10 @@ Output only valid JSON matching this structure:
             rows={[
               ["Arrow Up / Page Up", "Next channel"],
               ["Arrow Down / Page Down", "Previous channel"],
+              ["0–9", "Type a channel number and jump to it after 1.5 s (e.g. press 1 then 4 → channel 14)"],
               ["G", "Toggle the program guide"],
+              ["M", "Mute / unmute"],
+              ["F", "Toggle fullscreen"],
             ]}
           />