Feat(templates): support custom templates in remote vaults

[ajselzilic]

Summary

Adds full custom-template support for remote ZenNotes vaults.

Remote vault users can now create, list, read, edit, rename, delete, and use custom templates through the same Templates settings and template picker available for local vaults.

Closes #723

What changed

• Added authenticated server endpoints for template CRUD:
  • GET /api/templates
  • GET /api/templates/read
  • POST /api/templates/write
  • POST /api/templates/delete
• Added custom-template operations to the desktop remote server client and web HTTP bridge.
• Added supportsCustomTemplates capability negotiation so older servers continue to degrade gracefully.
• Enabled custom-template management in Settings when the connected remote server advertises support.
• Reloaded templates when .zennotes/templates changes through the server watcher.
• Ensured the templates directory exists before the watcher starts, preventing the first template write from being missed.
• Added safe path validation, atomic writes, note-size limits, duplicate-name handling, and bounded filenames.
• Updated remote-vault and security documentation.