[Bug] Enter key after choosing a file doesn't start upload (no form submit path)

I will pay for a fixed file - details below. ## Bug The picker and button sit loose in a `` with `onClick` handlers. Keyboard users who select a file and hit Enter expect submission; nothing happens because there is no wrapping `` and the button is `type="button"` with no submit association. Wrap the controls in a form with `onSubmit={(e) => { e.preventDefault(); handleUpload(); }}` and make Upload `type="submit"`. ## Acceptance criteria - [ ] Enter submits when a file is selected; guarded when none/in-flight - [ ] Page never reloads (preventDefault verified) - [ ] Double-submit guard from the related race-condition issue still effective through the submit path ## Reward I will pay **15 USDC (test tokens)** on **ARC Testnet** for a merged fix — same flow as #53, but payout is test USDC on ARC, not Base. How to claim: 1. Comment `/attempt` with this issue number before you start. 2. Open a PR referencing this issue (`Fixes #`). 3. After the PR is reviewed and merged, comment your **ARC testnet EVM address** here — you receive 15 test USDC on ARC.