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

TypeScript 5.x flags `baseUrl` as deprecated; it stops functioning
in TypeScript 7.0. Since TS 5.0, `paths` keys without a `baseUrl`
are resolved relative to the tsconfig file where they are declared
— in our case, the repo root, which is exactly what `baseUrl: "."`
was buying us. Removing it is a no-op for module resolution.

Our paths (`./libs/shared/tokens/src/index.ts`, …) already point at
files relative to the repo root, so this is a pure cleanup: no other
tsconfig in the workspace declares `baseUrl`, and the only path
imports across the codebase are the workspace aliases
(`shared-ui`, `shared-state`, `shared-tokens`, `shared-util`,
`feature-auth`) which keep resolving correctly.

`rootDir: "."` stays for now — 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.

Verified across portal-shell, portal-admin, shared-{ui,state,
tokens,util} with lint + test + build — green.

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"],