Servers: online player list with skin heads in Management tab

[kytxii]

Shows online players with their Minecraft skin heads in the Management tab. Clicking a player pre-fills the RCON console input.

Requirements

• Player list sourced from state.players (array of usernames from the management state poll)
• Each player shows their Minecraft head from https://mc-heads.net/avatar/{username}/32
• Clicking a player sets the console input to kick {username} (trailing space, cursor at end)
• "No players online" empty state
• List updates automatically as management state polls every 15s
• Image load failure: show {username[0].toUpperCase()} in a zinc-700 circle as fallback

Files to touch

• frontend/src/pages/ManagementTab.jsx

Definition of done

• Player heads render
• Clicking a player pre-fills the RCON console correctly
• "No players online" empty state shown
• Image fallback (letter avatar) works