Add page templates

## Goal

Cortext starts every new page from blank. Add page templates so creators can start from a layout that already fits what they're trying to write, instead of staring at an empty document.

## Scope

* Per-workspace page templates with a title and starter block content.
* Pick a template when creating a new page from the sidebar.
* Manage templates from the workspace: create, edit, rename, duplicate, delete.
* Optionally choose a default template for the sidebar's "New page" action.

## Out of scope

* Row templates; #111 covers those.
* Templates with conditional logic.
* Cross-workspace template sharing.

## Acceptance criteria

* You can create a page template and apply it when creating a new page.
* Templates carry a title and block content; both copy correctly into the new page.
* Editing a template doesn't retroactively change pages that were created from it.

Start by tracing the sidebar’s “New page” flow and the existing workspace, page, and block-content handling. The work is done when workspace templates can be created, edited, renamed, duplicated, deleted, optionally selected as the default, and applied with their title and blocks copied without later template edits changing existing pages.

Written by the indexing model from the issue text.