Event creation should save the image locally instead of only linking the external URL

[hhkaos]

When creating an event with an image, the editor stores/references the external image URL as-is. If the source removes or changes it later, the event's `image` breaks silently.

The event-creation flow should download the image and save a local copy (e.g. into the target repo's assets) as part of the propose-change/direct-edit flow, so the published event doesn't depend on a third party's URL staying alive.

[hhkaos]

Implemented in 8e5a27d, opt-in as requested.

What changed

• The image repeater now has a per-image checkbox: "Save a local copy (I have the rights to host this image)", unchecked by default. It's editor-only UI state — it never appears in the published event JSON, the "Copy JSON" output, or the "Editar directo" flow.
• It only takes effect via "Proponer cambio" (issue → PR), the only write path with a server-side component that can actually download and commit a binary. "Editar directo" (github.dev) has no code execution of its own, so it's out of scope here — implementing it there would need a GitHub API auth flow the editor deliberately doesn't have (see DESIGN.md: "cero backend, cero auth").
• When checked, the checked URLs ride as a transient _localizeImages key on the issue's JSON block. issue-to-pr.mjs downloads each one (https-only, capped/re-validated redirects, DNS-based rejection of private/loopback/link-local targets, size cap, timeout, content-type check — the issue author is anonymous and untrusted), commits it under assets/<slug>/, and rewrites the event's image url to raw.githubusercontent.com. _localizeImages itself is always stripped before the file is written, so it never reaches events/<slug>.json.
• A failed download never blocks the PR — that image just keeps its original external link, and a note is appended to the issue's sticky comment.
• No schema change needed: image[].url already accepts any absolute https:// URL.

Also fixed while in there: checkbox fields (All-day event, CFP travel/accommodation coverage, and the new one) were rendering label-above-checkbox like every other field — now they read [x] Label on one line.

Closing as done.