feat: add "Import as template" to File Tree context menu

[Adityakk9031]

Resolves #590

Description

This PR addresses the feature request to easily convert existing documents into templates. It adds an Import as template option directly to the right-click context menu of standard markdown files in the File Tree.

Users can now choose to:

• Keep original file: Copies the content and its frontmatter directly into a new template within .ok/templates/.
• Convert (delete original): Copies the file to the templates directory and safely removes the source document, updating the UI automatically.

Changes Made

• API (Core & Server): Added a secure POST /api/template/import endpoint (handleTemplateImport) which atomically extracts the source file's frontmatter, manages potential disk/sync conflicts, and persists the new template.
• Frontend (FileTree.tsx & Client API): Added the nested context menu items using the Lucide FileKey icon, bound them to a new importTemplate client wrapper, and handled immediate UI state cleanups (closing open tabs and removing deleted source files from the view).
• Changeset: Added a patch changeset for the release notes.

[CLAassistant]

All committers have signed the CLA.