Admin panel: Add/Edit mode, optional PYQ branch, and contributor visibility control

[Lav-developer]

Motivation

• Enable full admin edit workflows because the previous UI made editing PYQs, contributors and users awkward or effectively read-only by not exposing a focused edit flow.
• Allow some PYQs to include an optional branch in their canonical title because some papers require course+branch in the display string.
• Add explicit contributor visibility control so only approved contributors appear on the public site (no dummy entries).

Description

• Added an Action Mode selector (Add New / Edit Current) and entity chooser (PYQ / Contributor / User) to the admin quick-create area and a mode switcher that shows/hides forms and loads data when edit mode is selected (admin.html, admin.js).
• Added an optional Branch input to the PYQ add form and updated title generation to include branch when provided via buildPyqTitle(course, branch, semester, subject, session) and persist branch to Firestore when creating PYQs (admin.html, admin.js).
• Added a showOnWebsite field for contributors in the admin add/edit flows, persisted it to Firestore, and extended contributor edit prompts to allow toggling visibility (admin.html, admin.js).
• Updated public-site contributor loading to only query contributors where showOnWebsite == true and removed the placeholder "join" card so only real contributors are rendered (script.js).

Testing

• Automated JavaScript syntax checks using node -c admin.js and node -c script.js completed successfully.
• Verified the modified admin UI elements and JS functions are present and callable by inspecting the updated files (syntax checks passed).
• No automated unit tests exist for these UI flows; changes were validated with the above static checks.

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Codex Task

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