Add a share modal, renew sharing links when opened, and improve modal accessibility

[wajrock]

Summary

Clicking Share now opens a modal that explains how sharing works before anything leaves the browser. Sharing links are renewed when they are opened, and the promo modal is now usable with the keyboard.

What changed

Share modal

• Share opens a modal that states three things: the plan is encrypted in the browser and the key exists only in the link, the link stays available for 30 days after it was last opened, and anyone with the link can open the plan.
• Nothing is sent until the user clicks Copy link. The button then creates and copies the link, with a state for each step: Copy link, Creating link, Copied, and Try again if it fails.
• Reopening the modal, or sharing from a page opened through a sharing link, copies the existing link without storing the plan again.

Link renewal

• When a link is opened and its blob is more than a day old, the Worker rewrites it under the same id, which restarts the 30-day R2 lifecycle rule.
• Links now expire 30 days after they were last opened instead of 30 days after they were created.
• Renewal runs in the background, and the one-day threshold limits it to one R2 write per link per day, even if a link is reloaded in a loop.

Accessibility

• The focus handling now lives in a shared Modal component, used by both the share modal and the promo modal.
• Focus moves into the modal when it opens, Tab and Shift+Tab cycle through its controls, Escape closes it, and focus returns to where it was.
• Closing the demo video with Escape no longer closes the promo modal behind it.

Other

• The footer is separated from the PlanView with a larger gap.
• The FAQ, the Highlights, the error screen and the README describe the renewal.

Tests

60 tests pass, with 91% line coverage. New tests cover the share modal (nothing sent before the click, link reused, error message), keyboard focus in the modals, and closing the video without closing the promo.

[vercel]

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