feat(player): remember emulator core per-game

[gantoine]

Description
^{Explain the changes or enhancements you are proposing with this pull request.}

Just like the selected emulator core is saved in localStorage per-platform, this now also saves the selected core per-game, and falls back to the per-platform core when a game has no saved core.

Fallback chain: per-game core → per-platform core → first supported core.

Closes #2042

Changes:

• Player.vue (write, shared by v1 + v2): when a core is chosen, writes both player:${rom.id}:core (per-game) and player:${rom.platform_slug}:core (per-platform, kept as the default for new games on that platform). Removal clears both.
• v2/views/Player/EmulatorJS.vue (read): reads the per-game core first, falls back to the per-platform core, then the first supported core. Added a supportedCores validity check so a stale/unsupported saved core can't stick.
• console/views/Play.vue (read): added a gameCore storage ref and prefers it over the platform core, both validated against the supported list.

The v1 Base.vue read is intentionally left untouched to honor the v1 freeze; it still reads the per-platform key (unchanged behavior). The feature is fully functional in v2 and the console.

AI assistance disclosure: This change was implemented with AI assistance (PostHog Code / Claude). All code was reviewed before submission.

Checklist
^{Please check all that apply.}

• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

Screenshots (if applicable)

N/A

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Created with PostHog Code

[Copilot]

Pull request overview

Adds per-game persistence for the selected EmulatorJS core, while keeping the existing per-platform core as a fallback default, so different games on the same platform can automatically preselect different cores.

Changes:

• Persist chosen core to localStorage under both per-game (player:${rom.id}:core) and per-platform (player:${platform_slug}:core) keys when selecting a core in the shared player component.
• Update v2 EmulatorJS player view to prefer per-game core, then platform core, then first supported core (with supported-core validation).
• Update console Play view to prefer per-game core over platform core (with supported-core validation).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
frontend/src/views/Player/EmulatorJS/Player.vue	Writes/removes per-game core key in addition to existing per-platform key.
frontend/src/v2/views/Player/EmulatorJS.vue	Reads per-game core first, adds supported-core validation, then falls back.
frontend/src/console/views/Play.vue	Adds per-game core storage ref and prefers it over platform core during boot.

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[greptile-apps]

Greptile Summary

This PR adds per-game emulator core persistence. The main changes are:

• Per-game core storage in the shared player writer.
• Per-game-first core selection in the v2 player view.
• Per-game-first core selection in the console player path.
• Validation of saved cores against supported core lists.

Confidence Score: 4/5

The per-game core fallback and shared write path need fixes before merging.

• Unsupported per-game storage can skip a valid platform fallback.
• The shared player writer can overwrite game-specific storage when launched from the v1 path.
• The emulator still falls back to supported cores, so the issue is incorrect preference behavior rather than a boot failure.

frontend/src/v2/views/Player/EmulatorJS.vue, frontend/src/console/views/Play.vue, frontend/src/views/Player/EmulatorJS/Player.vue

Important Files Changed

Filename	Overview
frontend/src/console/views/Play.vue	Adds a per-game core storage ref and prefers it during console boot, but stale per-game values can bypass a valid platform fallback.
frontend/src/v2/views/Player/EmulatorJS.vue	Adds per-game-first core selection with validation, but the fallback chain skips platform validation when the game key is present and unsupported.
frontend/src/views/Player/EmulatorJS/Player.vue	Writes selected cores to both per-game and per-platform keys, which can overwrite game-specific storage from the v1 launch path.

_{Reviews (1): Last reviewed commit: "feat(player): remember emulator core per..." | Re-trigger Greptile}