feat(base)!: removing a profile makes it private (ADR 0020, D27)

Every account keeps a handle now, so removing a public profile no
longer deletes it and frees the handle: it takes away what the profile
shows and leaves it private, as a profile is until its person goes
public (D5).

- `profile.remove` and the register's `removeProfile` (members ask the
  register) empty the display name, bio, links with their checks and
  the avatar, keep the row with its handle, and return what is left,
  null for none. `/@handle` and `profiles.byHandle` then answer as for
  a handle nobody has. Once the person has published, removing is
  refused as before.
- Learn: the profile form starts afresh from what is left, "Remove
  profile" shows only while something is filled in, and the dialog says
  the handle stays.
- The module's README and ADR 0020 amended.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -585,7 +585,7 @@
   "publicProfileHandleConfirmChange": "Change",
   "publicProfileRemove": "Remove profile",
   "publicProfileRemoveTitle": "Remove your public profile?",
-  "publicProfileRemoveMessage": "Your handle becomes free for others to take.",
+  "publicProfileRemoveMessage": "Your display name, bio, links and picture are removed. Your handle stays yours and private until you fill in your profile again or publish something.",
   "publicProfileRemoved": "Profile removed.",
   "publicProfileFailed": "Couldn't reach the instance. Try again.",
   "publicProfileUnreachable": "Your public profile can't be reached right now.",