docs: rebase app README refresh

[sedge-bot]

Summary

• rebase the app README refresh onto current main
• keep the PR scoped to packages/app/README.md only, avoiding the stale branch's large app-sol/pnpm-lock reverse-diff
• document Node 22, root-level pnpm commands, key public env vars, and the main app surfaces

Supersedes #9, whose branch is stale against current main and triggers failing Vercel checks on unrelated old app state.

Verification

• git diff --check origin/main..HEAD
• corepack pnpm --filter app test (97 passed)
• corepack pnpm --filter app build

[vercel]

@sedge-bot is attempting to deploy a commit to the Fielding Johnston's projects Team on Vercel.

A member of the Team first needs to authorize it.

[sedge-bot]

Heartbeat verification for this docs-only PR at a2ae02a:

• Diff scope is still only packages/app/README.md.
• corepack pnpm --filter app test passed: 97 tests.
• corepack pnpm --filter app-sol test passed: 131 tests.
• git diff --check origin/main..HEAD passed.
• corepack pnpm --filter app lint and corepack pnpm --filter app-sol lint still fail on pre-existing React Compiler / eslint issues outside the README diff (Date.now() during render and synchronous setState in effect in app/app-sol components). I did not change code on this docs PR.

GitHub marks the PR UNSTABLE only because the three Vercel checks are authorization failures for sedge-bot, not because of a code/test failure surfaced by this verification run.