feat(feedback): implement PyCon 2026 feedback form

[georgegale228]

Add the feedback page for PyCon 2026 featuring an anonymous experience survey form, client-side validation, programmable locking.

Changes

• New page app/templates/2026/2026_feedback.html : full form with input fields, radio buttons, checkboxes, and textareas.
• Dedicated CSS app/static/2026/css/pages/feedback.css : responsive layout and locked/unlocked state styling.
• JavaScript app/static/2026/js/feedback.js : validation, AJAX submission, and form open/closed state management.
• Router app/routers/router_2026.py : exposes the template and the feedback_is_open flag.
• Updated base.html, 2026_partners.html, and 2026_speakers.html for page integration.

Behavior

• The form is hidden/disabled as long as feedback_is_open is false.
• Informational message displayed with the scheduled opening date (August 28, 2026, 12:00 PM UTC).

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