chore(ts): drop deprecated baseUrl from tsconfig.base.json (#101)

## Summary

Remove the deprecated `baseUrl: "."` from [`tsconfig.base.json`](tsconfig.base.json). TypeScript 5.x flags it as deprecated; it stops functioning in TS 7.0. Since TS 5.0, `paths` keys are resolved relative to the tsconfig file where they are declared when `baseUrl` is absent — and the repo root is exactly what `baseUrl: "."` was pointing at.

## Why this is a no-op

Our `paths` entries already start with `./libs/shared/.../src/index.ts` (relative to the repo root, which is `tsconfig.base.json`'s directory). With or without `baseUrl: "."`, the resolver lands on the same files. No other tsconfig in the workspace declares `baseUrl` (only `tsconfig.base.json` did), and the only path imports across the codebase are the workspace aliases (`shared-ui`, `shared-state`, `shared-tokens`, `shared-util`, `feature-auth`).

`rootDir: "."` stays — it is independent of the deprecation and removing it would change `dist/` layout semantics across the workspace. Revisit only if a future TS version flags it.

## Verification

`pnpm exec nx run-many -t lint test build --projects=portal-shell,portal-admin,shared-ui,shared-state,shared-tokens,shared-util` — green across 6 projects.

The IDE deprecation warning is gone.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #101

tsconfig.base.json

@@ -1,7 +1,6 @@
 {
   "compilerOptions": {
     "rootDir": ".",
-    "baseUrl": ".",
     "paths": {
       "shared-tokens": ["./libs/shared/tokens/src/index.ts"],
       "shared-util": ["./libs/shared/util/src/index.ts"],