Fix: prevent joining games already in progress

[xovishnukosuri]

Summary

Fixes #2521

• Players could join a game after it had started by navigating directly to /games/:game_id/players/new, allowing them to watch the game in real time and vote on cards without being dealt any cards themselves.
• Added a started_at check in PlayerLive.Index.apply_action(:new, ...) to redirect users away from the join form if the game has already started.
• Added a defense-in-depth started_at check in PlayerLive.FormComponent.save_player(:new, ...) to reject player creation even if the form was loaded before the game started (race condition).

Changes

• copi.owasp.org/lib/copi_web/live/player_live/index.ex — check game.started_at in apply_action(:new, ...) and redirect with error flash
• copi.owasp.org/lib/copi_web/live/player_live/form_component.ex — re-fetch game in save_player(:new, ...) and reject if started_at is set

Test plan

• Create a game, add 3+ players, start the game
• Try navigating to /games/:game_id/players/new — should be redirected to the game page with an error flash
• Verify existing join flow still works for games that have not started
• Verify edge case: open join form, then start game in another tab, then submit join form — should be rejected

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