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

[sh1vam31]

@nishantwrp, Kindly review this PR.

[alondmnt]

Thanks for tackling this @sh1vam31. The approach and error messaging look good for the tag-based workflow.

One issue: isNoteATemplate hardcodes a check for the template tag, but when TemplatesSource is set to "Notebook", templates live in a "Templates" folder and don't need that tag. Since getTemplateFromId is shared, this would cause notebook sourced default templates to be incorrectly rejected.

The fix should respect TemplatesSourceSetting for notebook source, check that the note is in a "Templates" folder instead of checking for the tag.