[feat] Support custom templates in remote vaults

[ajselzilic]

Description

Custom templates currently work only with local desktop vaults. In the self-hosted web client and desktop remote-workspace mode, Settings reports that custom templates require a local vault, even though the remote server owns a normal filesystem vault.

Built-in templates continue to work because they are bundled with the client, but custom templates stored under .zennotes/templates/ cannot be listed, created, edited, or deleted remotely.

Desired behavior

Custom templates should work when:

• using the self-hosted web client
• connecting the desktop app to a remote ZenNotes server

Templates should remain stored in the active vault’s .zennotes/templates/ directory, preserving the existing template format and UI.

Changes made by one connected client should become visible to other clients without requiring a reconnect.

Proposed approach

• Add authenticated server endpoints for listing, reading, writing, and deleting custom templates.
• Advertise support through an optional server capability flag so older servers remain compatible.
• Use those endpoints from both the web client and desktop remote-workspace client.
• Report template-directory changes through the existing vault watcher.
• Keep local-vault template behavior unchanged.

Security and compatibility

Remote template operations should:

• use the server’s existing authentication/session middleware
• restrict paths to flat Markdown files under .zennotes/templates/
• reject path traversal
• use the configured note-size limit
• use the vault’s existing locking and atomic-write behavior
• remain disabled when an older server does not advertise support
• leave built-in templates available on older servers

Acceptance criteria

• Custom templates can be created, edited, listed, read, and deleted from the self-hosted web client.
• The same operations work in desktop remote-workspace mode.
• Template changes refresh across connected clients.
• Older servers degrade gracefully without breaking built-in templates.
• Local-vault template behavior is unchanged.
• Server authentication, path validation, size limits, CRUD, watcher events, and UI capability gating are covered by tests.

Out of scope

• Changing the custom-template syntax or frontmatter format.
• Redesigning the existing template editor or picker.
• Synchronizing templates between different vaults.

Prior work

This is a focused follow-up to #592.

@flokchvtr previously explored server/web custom-template support in the web-feature-parity branch, including commit 821b816.

The template portion does not appear to have a focused pull request yet. I am opening this issue first, as requested by the contribution guide, to confirm the scope and avoid duplicating ongoing work.