feat: add clear cache button to publish - #494

Summary

Adds a "Clear cache" button to the publish popover so users can force a full cache purge of their published website. It appears next to the Publish/Update button only once the site has been published, and is guarded by a confirmation dialog that explains when and why it's needed.

Changes

• Add a clear-cache icon button beside the Publish/Update button in PublishPopover, shown only when the site is already published
• Wire it to the existing cacheApi.clearAll() helper (POST /ycode/api/cache/clear-all) — no new backend code
• Add a confirmation dialog explaining that clearing is normally unnecessary (publishing already refreshes changed pages), is mainly useful for websites with many pages, and that the first visit to each page is slower until it's cached again
• Reuse existing patterns: spinner/success states, toast feedback, and a tooltip for the icon-only button

Test plan

• On an unpublished site, confirm only the Publish button shows (no clear-cache button)
• After publishing, confirm the clear-cache button appears beside Update
• Click it and confirm the dialog opens with the explanatory copy
• Confirm "Clear cache" purges the cache (success toast) and the dialog blocks interaction while in progress
• Confirm "Cancel" closes the dialog without clearing