Add post sort control to template editor

[davidmerfield]

Motivation

• Ensure an index-page post-sorting control is shown even when a template's locals.sort_by/locals.sort_order are missing by providing sensible defaults.
• Provide a single, canonical set of select options and labels for index sorting so the loader and saver share the same mapping.
• Persist both sort_by and sort_order when the select is submitted so templates receive complete sorting metadata.

Description

• Add app/dashboard/site/template/sort-options.js with the four required options and their exact labels and mapping values.
• Update app/dashboard/site/template/load/index-inputs.js to always append a select-style control (matching existing select rendering) named sort_by, using defaults sort_by: "date" and sort_order: "desc" when locals are absent.
• Implement selection resolution so the loader picks the correct option value from the shared options list and marks it selected.
• Update app/dashboard/site/template/save/layout-inputs.js to map the submitted select value back into both req.locals.sort_by and req.locals.sort_order, falling back to the same defaults when the submitted value is missing or invalid.

Testing

• No automated tests were run.

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Codex Task