Close the CFP and remove submit links

[cemrehancavdar]

The CFP deadline (7 September 2026, 23:59 AoE) has passed, so this removes every submit link and flips the copy to "closed". Tickets become the primary call to action.

What changed

• Hero: removed the "Submit a talk! 🎤" button; the ticket button is now the sole CTA (builds on the existing "Add Get a ticket button" commit).
• Homepage: removed the floating submit CTA and its scroll observer; section title is now "Call for Proposals is Closed".
• event.ts: dropped the unused cfpUrl, cfp.url, and cta submit object (kept cfp.deadlineDisplay).
• CFP copy (cfp.mdx, cfp-details.mdx): rewritten to closed messaging; removed the "Submit your talk" link/banner.
• /cfp/ page: removed the submit button and updated the meta description; page kept as an archive.
• global.css: removed the now-dead .cta-hidden rule.

The CFP nav item and /cfp/ page are intentionally kept as an archive showing the closed status. No forms.gle/submit links remain.

Verification

• astro check: 0 errors / 0 warnings
• astro build: succeeds (7 pages)
• Manually verified home and /cfp/ in a browser: no submit links, correct closed messaging.

Note: this branch also carries the prior 25ecf1f Add Get a ticket button commit, since the hero now relies on it as the primary CTA.

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📚 Documentation preview 📚: https://python-pizza-istanbul--10.org.readthedocs.build/

[egeakman]

Closing in favor of #11