[EPIC](apps): Migrate to Clean TypeScript ESLint Configuration

[ArtieReus]

Description

Our current ESLint setup has accumulated technical debt from the JS-to-TS migration:

Problem: packages/config/eslint/juno-typescript.mjs mixes JavaScript and TypeScript rules (lines 35-36 use both pluginJs.configs.recommended and tseslint.configs.recommendedTypeCheckedOnly), causing rule conflicts. Each app works around this by disabling 15-20 TypeScript rules.

Solution: Create a clean, Vite-based TypeScript ESLint configuration (vite-react-ts.mjs) and migrate each app independently. This allows parallel work without coordination overhead and eliminates JS/TS rule interference.

Why it matters:

• Improves type safety by re-enabling strict TypeScript checks
• Better IDE support and autocomplete
• Clearer distinction between intentional overrides vs. workarounds

Objectives

• Eliminate JavaScript/TypeScript rule conflicts in shared config
• Enable strict TypeScript checking across all apps (re-enable currently disabled rules)
• Allow independent, parallelizable migration work per app
• Improve developer experience with better type safety and IDE support
• Establish clear ESLint conventions for future development

Acceptance Criteria

• New packages/config/eslint/vite-react-ts.mjs config exists with pure TypeScript rules
• All 7 apps migrated: carbon, doop, greenhouse, heureka, supernova, example, template
• All 8 packages migrated: oauth, ui-components, greenhouse-auth-provider, template, communicator, k8s-client, url-state-provider, messages-provider
• Each app/package has ≤5 rule overrides, all with justification comments
• All apps and packages pass pnpm lint with new config (CI green)
• Old juno-typescript.mjs and juno.mjs removed from packages/config
• Migration documented in CLAUDE.md or ADR
• Template PR created showing migration pattern for other apps/packages to follow

Tasks

Phase 1: Foundation

• Research and document Vite's default ESLint setup for React-TS
• Create packages/config/eslint/vite-react-ts.mjs with pure TypeScript rules
• Export new config from @cloudoperators/juno-config package
• Document baseline configuration

Phase 2: Migration

Each app/package follows this workflow:

1. Switch to new config in eslint.config.mjs
2. Remove all custom rule disables
3. Fix violations category-by-category (separate PRs per category)
4. Document any necessary exceptions with justification comments

Apps migration (simplest → most complex):

• Migrate apps/example (template PR for others to follow)
• Migrate apps/template
• Migrate apps/doop
• Migrate apps/heureka
• Migrate apps/supernova
• Migrate apps/greenhouse
• Migrate apps/carbon

Packages migration (parallelizable with apps):

• Migrate packages/communicator
• Migrate packages/k8s-client
• Migrate packages/messages-provider
• Migrate packages/oauth
• Migrate packages/template
• Migrate packages/url-state-provider
• Migrate packages/greenhouse-auth-provider
• Migrate packages/ui-components (likely most complex, has Storybook plugin)

Phase 3: Cleanup

• Verify all apps and packages using vite-react-ts.mjs
• Remove packages/config/eslint/juno-typescript.mjs
• Remove packages/config/eslint/juno.mjs (if unused)
• Update packages/config/package.json exports
• Update CLAUDE.md with new ESLint guidelines
• Create ADR documenting the migration decision

Dependencies

• Supersedes original epic [EPIC](apps): Migrate to Clean TypeScript ESLint Configuration #1565 (different approach to same problem)
• No external dependencies - can start immediately
• No blocking dependencies between app/package migrations (fully parallelizable)
• Note: packages/ui-components uses eslint-plugin-storybook - the new config must be compatible

Additional Notes

Strategy: Create new config alongside old one, migrate apps independently, then remove old config.

Per-app workflow details:

1. Switch app's eslint.config.mjs to import vite-react-ts.mjs
2. Remove all custom rule disables
3. Run pnpm --filter {app} lint to capture violations
4. Fix violations by category (easy → medium → hard):
   • Easy: no-unused-vars, prefer-const, no-extra-boolean-cast
   • Medium: @typescript-eslint/no-unsafe-assignment, no-floating-promises
   • Hard: @typescript-eslint/no-unsafe-member-access, no-unsafe-call
5. Each category = separate PR
6. Document necessary exceptions with justification comments

Risk mitigation:

• Old config stays until all apps migrated (rollback possible)
• App-specific overrides allowed with mandatory justifications
• Feature branches prevent CI blocking unrelated work
• Template PR from apps/example provides pattern for team

Technical details:

Current problematic config structure:

// packages/config/eslint/juno-typescript.mjs (lines 35-36)
pluginJs.configs.recommended,  // JS rules
...tseslint.configs.recommendedTypeCheckedOnly,  // TS rules
// ☝️ These conflict with each other

Target clean structure:

// packages/config/eslint/vite-react-ts.mjs
...tseslint.configs.recommendedTypeChecked  // Only TS rules
// No JS/TS mixing

Questions to resolve before starting:

1. Should we use Vite's exact ESLint config or customize slightly?
2. Enforce stricter rules than Vite defaults (e.g., strict: true)?
3. Which app for template/pilot PR (recommend apps/example)?
4. ADR or just update CLAUDE.md?