fix profile management — duplicate prevention, debounce, delete from view

1. Duplicate nsec check: if the key already exists in another profile,
   shows error with the profile name. Prevents accidental duplicates
   from rapid clicking.

2. Save button debounce: disabled after first click, re-enabled on
   completion or error. Prevents multiple profile creation on
   unresponsive UI.

3. Delete from profile view: "Delete This Profile" button now visible
   on the read-only profile view screen (not just the edit screen).
   Only shows when there's more than one profile.

Author: vveerrgg