fix(ui): expose importable JavaScript entrypoint for @freelanceflow/ui

[RanuK12]

Summary

• Add a proper JavaScript entrypoint to @freelanceflow/ui so workspace consumers can import the package at runtime.
• Update packages/ui/package.json: main → dist/index.js, add types, exports, build script, and peerDependencies.
• Add packages/ui/tsconfig.json with JSX support for compiling React components.
• Add regression tests proving the package is directly importable and exports Button and Card.

Problem

@freelanceflow/ui declared main as src/index.ts, a TypeScript source file. Node.js cannot resolve TypeScript directly, so import("@freelanceflow/ui") fails at runtime. The component modules (Button, Card) were not reachable as JavaScript.

Fix

Following the same pattern that was applied to @freelanceflow/db (#2775), this PR:

1. Adds tsconfig.json targeting ES2020/CommonJS with jsx: "react" for TSX compilation.
2. Updates package.json with proper main, types, and exports fields pointing to dist/.
3. Adds a build script (tsc) and TypeScript dev dependency.
4. Adds react as a peer dependency (required by the components).

Validation

• node --test packages/ui/test/ui-entrypoint.test.js — 4/4 pass
• node --test apps/api/src/tests/*.test.js — 1/1 pass (no regressions)
• Manual verification: node -e "import(./packages/ui/dist/index.js).then(m => console.log(Object.keys(m)))" → ["Button", "Card"]

/claim #743
Closes #4168