Upgrade to typed linting with projectService and recommendedTypeChecked

[lee-chase]

Context

Identified during review of #619 (feat/ts-support), which establishes the TypeScript foundation for this project.

Target

Next branch (typescript)

Current state

The ESLint config uses the legacy split packages (@typescript-eslint/eslint-plugin + @typescript-eslint/parser) with project: './tsconfig.json' and only recommended rules — meaning we are paying the cost of typed linting without using any type-aware rules.

Target state

Once the JS → TS migration is substantially complete on the next branch:

1. Replace split packages with the unified typescript-eslint package
2. Switch project: './tsconfig.json' → projectService: true, tsconfigRootDir: import.meta.dirname
3. Upgrade TS block from recommended → recommendedTypeChecked to enable type-aware rules (no-floating-promises, no-misused-promises, await-thenable, etc.)
4. Add tseslint.configs.disableTypeChecked to the JS block for any remaining .js config files

Risks

• New lint errors — recommendedTypeChecked will surface violations on existing .tsx files that must be fixed in the same pass
• allowDefaultProject — any .js files retained in tsconfig.json's include array will need this option to avoid parser errors
• Cache invalidation — first lint run will bypass the ESLint cache

Dependency

Should be done after the majority of JS files have been converted to TypeScript.