Unify plugin install prompts and revamp the add plugin modal

[Salvialf]

Fixes #3418, and while at it, unifies plugin install feedback regardless of source.

• Replace the Cancel/OK buttons shown after installing a plugin from the Market with Yes/No, and reuse the same prompt for plugins installed from any other source (file, GitHub, URL, Samba), which previously just showed a toast and left the user with a stale plugin list.
• Rework the "Add a plugin from another source" modal: tooltips, placeholders instead of prefilled values, a Stable/Beta dropdown for Market installs, and restrict the manual file upload to .zip.
• Harden the GitHub source: surface curl failures instead of silently producing an empty archive, fall back to sensible defaults when user/repository/version are left blank, and fix an ordering issue so the installed commit SHA is captured on the very first install.
• Remove dead code found along the way (an unused image-type switch and a call to a function that was never defined).
• Fix an orphaned database row left behind when adding a new plugin from another source fails partway through.
• Apply the same orphaned-row cleanup to the Market's visual "Install" button (repo.ajax.php), which had the identical gap.
• jee_modal was oversized for the content of this modal. Switch to jee_modal3 (60vw) instead