Add reusable collection row templates

## Goal

Let collections define reusable starting points for new rows.

## Scope

* Template management on a collection: create, edit, rename, duplicate, and delete templates.
* A template carries row title, property defaults, and `post_content` block content.
* A per-view default template that the New action uses.
* A template picker on the New action when a collection has more than one template.

## Out of scope

* Page templates.
* Repeating templates on a schedule.
* Templates with conditional logic.
* Cross-collection template sharing.

Start by locating the collection row creation flow, the New action, and per-view settings; the issue provides no file or test names. Trace how row titles, property defaults, and post_content are currently initialized. Done means collection-level template management, per-view defaults, and the multi-template picker work while the listed out-of-scope cases remain unsupported.

Written by the indexing model from the issue text.