chore: remove deprecated baseUrl from client tsconfig — migrate to paths-only resolution

[NickMonrad]

Background

baseUrl: "." in client/tsconfig.app.json is deprecated in TypeScript 6 and will stop working in TypeScript 7. Added "ignoreDeprecations": "6.0" as a temporary suppression in PR #212 (TS6 upgrade).

What needs to change

1. Remove "baseUrl": "." from client/tsconfig.app.json
2. Remove "ignoreDeprecations": "6.0" (no longer needed)
3. Add a Vite alias in client/vite.config.ts so the @/* import path still resolves at build time:

   import path from 'path'
   // in defineConfig:
   resolve: {
     alias: { '@': path.resolve(__dirname, './src') }
   }

4. Verify @/ imports still work in tests (client/src/test/TaskList.test.tsx, projectCustomer.test.ts) — Vitest may need a matching alias in vitest.config.ts or the Vite config is shared automatically
5. Run tsc --noEmit -p tsconfig.app.json, vite build, and client tests to confirm

Why deferred

The paths mapping in tsconfig gives TypeScript type-resolution for @/ imports. Without baseUrl, the anchor for relative paths shifts — this needs careful testing to ensure no import breaks at runtime or in tests. Low risk but deserves its own focused PR rather than bundled into the TS6 upgrade.