[codex] Add Epic account switching

[colonelpanic8]

Fixes #5456.

Summary

This adds multi-account support for Epic/Legendary without replacing the currently saved account while logging in to another one.

• store Epic accounts separately from Legendary's active profile and track the active account in Heroic config
• add Login-page UI to view saved Epic accounts, switch the active account, add another account, and remove inactive accounts
• run add-account login in an isolated temporary Legendary config, then promote it only after auth succeeds
• preserve and self-heal Legendary install metadata when switching accounts so installed games stay recognized
• add a Nix flake dev environment for setup, start, lint, tests, and the combined check workflow

Validation

• corepack pnpm lint passed with warnings only; no lint errors
• corepack pnpm exec eslint --cache . --quiet passed
• corepack pnpm prettier passed
• corepack pnpm find-deadcode passed
• corepack pnpm codecheck passed
• corepack pnpm test:ci passed: 22 suites, 142 tests
• corepack pnpm exec electron-vite build passed, with existing Rollup/chunk-size warnings
• nix run path:/home/imalison/Projects/HeroicGamesLauncher#check --extra-experimental-features 'nix-command flakes' --no-update-lock-file passed
• git diff --check and cached diff whitespace checks passed
• Manually added a second Epic account, switched to it, refreshed the library, preserved the previous account, and launched Rocket League successfully under the new active account

Checklist

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

Notes on unchecked items: manual validation was done against an existing developer install so we could verify install-metadata preservation and launching an already-installed Rocket League copy. Login, account switching, library refresh, and play were manually tested; install, uninstall, and move were not manually exercised.

[flavioislima]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[colonelpanic8]

I have read the CLA Document and I hereby sign the CLA