Feedback report form can't attach a screenshot/image

A beta tester filing a UI-glitch report wrote: "I have a screenshot but no way to attach it to this report." Visual bugs are common and a screenshot is often the fastest way to convey them.

**Current state:** the in-app feedback modal (`app/src/renderer/index.html` report form -> `app/src/renderer/app.ts` submit handler) collects only a title, a body, and optional auto-collected diagnostics (sysinfo + activity/shell tails). The `FeedbackPayload` contract (`shared/feedback-contract.d.ts`) has no attachment field, and the GitHub-issue fallback path sends title + body only. The body placeholder even invites "screenshots if relevant," but there's no actual way to attach one.

**Ask:** let users attach one or more images (file picker and/or paste-from-clipboard) to a feedback report. Requires a new field on the feedback payload contract plus handling on the receiving service. Consider an image-appropriate size limit (the current payload is capped at 200 KB, which is too small for screenshots -- images likely need a separate, larger cap or an out-of-band upload), and decide how the GitHub-fallback path behaves when it can't carry attachments.

Surfaced alongside the copy-button bug #299, where the tester specifically wanted to attach a screenshot.