Upgrade eslint to latest (with flat config update)

[seriouslysean]

@itsalaidbacklife depending on which of these PRs merge first, I'll probably have to rejigger a few things to align the dependencies. See these:

• Chore/unused deps 20251028, resolves #1279 #1280
• Upgrade vite & vitest #1282

Issue number

Relevant issue number

• Resolves #

Please check the following

• Do the tests still pass? (see Run the Tests)
• Is the code formatted properly? (see Linting (Formatting))
• For New Features:
  • Have tests been added to cover any new features or fixes?
  • Has the documentation been updated accordingly?

Please describe additional details for testing this change

[Copilot]

Pull Request Overview

This PR upgrades ESLint from v8.54.0 to v9.38.0 and migrates to the flat configuration format, addressing the deprecation of the legacy .eslintrc.js format. The changes include updating ESLint plugins, refactoring the configuration file, and applying formatting fixes to ensure code compliance with the new linter setup.

• Upgraded ESLint to v9.38.0 and related plugins to compatible versions
• Migrated from .eslintrc.js to the new eslint.config.js flat config format
• Applied formatting fixes across the codebase for compliance with updated linting rules

Reviewed Changes

Copilot reviewed 7 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
package.json	Updated ESLint and related plugin dependencies to v9-compatible versions
package-lock.json	Locked dependency versions reflecting the ESLint ecosystem upgrade
eslint.config.js	New flat config file replacing the deprecated .eslintrc.js format
.eslintrc.js	Removed legacy ESLint configuration file
vitest.config.mjs	Applied formatting fixes for array bracket spacing
tests/e2e/support/helpers.js	Reformatted multi-line conditional for readability
tests/e2e/plugins/index.js	Removed unused eslint-disable comment
src/stores/game.js	Refactored computed properties for improved readability and formatting
.storybook/preview.js	Updated quotes to single quotes per linting rules
.storybook/main.js	Applied formatting fix for array bracket spacing

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