fix: validate default template still has template tag before using it

[sh1vam31]

Fixes #107

If a note was set as the default template and later had its template tag removed, the plugin would silently fail or give a misleading error. It should instead inform the user that the saved default is no longer a template.

Fix

src/utils/templates.ts

• Added isNoteATemplate(noteId) — checks if a note still has the template tag
• Updated getTemplateFromId() to return null if the note is no longer a template

src/index.ts

• Both "Create note from default template" and "Create to-do from default template"
  commands now show a clear, actionable error message:

"The note set as the default template is no longer a template.
Please set a new default template via Tools → Templates → Default templates."

Testing

1. Created a note, tagged it as template, set it as the default template
2. Removed the template tag from that note
3. Clicked Tools → Templates → Create note from default template
4. The correct error message appeared
5. Re-added the template tag → default template worked normally again