feat: players page for everyone, with player details, playtime and sessions

[BK1031]

• GET /accounts now attaches stats: {playtime_minutes, sessions} to each account from warden_player_stats, in one query for the whole roster; omitted for players the plugin hasn't reported on, and a stats query failure leaves the listing intact
• Player cards show Playtime (42m, 12h 5m, 1,240h) and Sessions next to "Seen", or — with no stats yet
• GET /accounts is open to any signed-in member (was MinecraftAdmins only), so every member sees the roster: Minecraft name/UUID, Sentinel name and username, last seen, playtime and sessions
• Players link shows for everyone in the sidebar and mobile menu
• Unlink button removed from roster cards (self-unlink in Settings and DeleteAccount are unchanged)
• Roster cards link to a new /players/:uuid page showing that player's stats in the same layout as Home (PlayerStatsView, now shared with Home); handles no-stats-yet and unknown UUIDs
• GET /accounts/:uuid (now with Sentinel identity) and GET /stats/:uuid are open to any signed-in member, matching the roster
• Last seen is relative under a day (5m ago, 23h ago), and the roster is sorted most recently seen first