chore: migrate findable-ui eslint to flat config (#926)

[frano-m]

Summary

Closes #926

Migrates findable-ui's ESLint configuration from the legacy .eslintrc.json + .eslintignore to flat config (eslint.config.mjs). Prerequisite for #932 (eslint-config-next v16, which requires flat config).

Approach

• New eslint.config.mjs that uses FlatCompat (@eslint/eslintrc + @eslint/js) to wrap legacy plugin extends — next, storybook-recommended, sonarjs-recommended, eslint-comments-recommended, prettier, @typescript-eslint/recommended.
• Top-level overrides (@typescript-eslint/explicit-function-return-type with .styles.ts(x) exclusion, no-explicit-any off for tests) preserved as flat-config blocks with files/ignores keys.
• .eslintignore migrated to ignores: ["**/node_modules/**", "**/out/**", "**/.next/**", "lib/**"] in the first config block.
• Stays on ESLint 8.57 with ESLINT_USE_FLAT_CONFIG=true env-var prefix on the lint script. The ESLint 9 bump and the cascading plugin updates (sonarjs, perfectionist swap, eslint-comments fork, sort-destructure-keys bump) are scoped to [P1.7] Next.js 16 prep: bump eslint-config-next + @next/eslint-plugin-next to v16 #932.

One pre-existing issue surfaced

.storybook/main.ts had a sort-keys violation that legacy ESLint silently allowed — legacy auto-ignored dot-directories by default. Flat config doesn't, so the violation now triggers. Fixed with a one-line key reorder (no behavior change).

Verification

• npm run lint exits 0 (only the pre-existing informational "Pages directory not found" warning).
• npm run check-format passes.
• npx tsc --noEmit passes.
• npm test passes (50/50 suites, 427/427 tests).

Follow-ups (out of scope for this PR)

• [P1.7] Next.js 16 prep: bump eslint-config-next + @next/eslint-plugin-next to v16 #932 absorbs the ESLint 9 bump, the env-var prefix drop, and the cascade of plugin updates (sonarjs 0.25 → ^1, typescript-sort-keys → perfectionist, eslint-comments → community fork, sort-destructure-keys v2 → v3, plus minor freshness on @typescript-eslint/eslint-plugin, eslint-plugin-jsdoc, eslint-plugin-react-hooks).

Part of #884.

[Copilot]

Pull request overview

Migrates this library’s ESLint setup from legacy .eslintrc.json / .eslintignore to ESLint flat config (eslint.config.mjs) while staying on ESLint 8 (using ESLINT_USE_FLAT_CONFIG=true)—a prerequisite step for the later eslint-config-next v16 / ESLint 9+ upgrade path.

Changes:

• Added eslint.config.mjs using FlatCompat to mirror the existing rule set and overrides.
• Updated the lint script and added required devDependencies for flat config (@eslint/eslintrc, @eslint/js).
• Removed legacy ESLint config files and fixed one .storybook/main.ts key-order violation exposed by flat config.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
package.json	Updates lint script to enable flat config under ESLint 8; adds flat-config helper devDependencies.
package-lock.json	Locks new @eslint/eslintrc / @eslint/js dependency graph changes.
eslint.config.mjs	Introduces the flat ESLint configuration (via FlatCompat) and preserves key overrides.
.storybook/main.ts	Reorders object keys to satisfy sort-keys now that dot-directories are linted.
.eslintrc.json	Removed legacy ESLint configuration.
.eslintignore	Removed legacy ignore file; ignores are now in eslint.config.mjs.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 6 changed files in this pull request and generated no new comments.