feat: public profiles, a handle on the account domain with name, bio, links and picture

Stage 1 of the public tier (ADR 0005). A person with an identity has a
public profile: a handle by Mastodon's rules (1 to 30 letters, digits or
underscores, unique whatever its case, a few kept for the instance),
changeable until they first publish, on the instance's account domain
(ACCOUNT_DOMAIN, else the server's host; the instance info names it), so
`raisondetredev@halkyon.eu`; a display name, a bio, up to four links and
an avatar. The table is server only: the app gets a Profile without the
person's id. ProfileEndpoint (identified only) reads, saves and removes
the caller's own; ProfilesEndpoint finds anyone's by handle.

An avatar is a PNG of at most 1 MB that the app draws anew from the
picture chosen, its middle square at 400 pixels, so a photo's metadata
never leaves the device. The server keeps it in the public storage named
by its content, so its URL tells nothing about whose it is, and deletes it
once no profile shows it. Deleting a person deletes their profile and its
picture.

In the app, the Profile tab holds the profile as a form, laid out as the
old app's; anonymous people are told GitHub sign-in gives them one.

Migration 20261009184924904-public-profiles creates public_profile with a
unique index on the person and one on the handle in lower case; it touches
no other table, and the old server never sees it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -1389,6 +1389,131 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get profileSwitchConfirm => 'Switch';
 
+  @override
+  String get publicProfileTitle => 'Public profile';
+
+  @override
+  String get publicProfileIntro =>
+      'How others see you once you publish: the handle they find you by, your name, a few words and links. Nothing here shows until you save it.';
+
+  @override
+  String get publicProfileAnonymousHint =>
+      'Sign in with GitHub to have a public profile and publish.';
+
+  @override
+  String get publicProfileDisplayName => 'Display name';
+
+  @override
+  String get publicProfileDisplayNameHint =>
+      'The name shown beside your handle';
+
+  @override
+  String get publicProfileHandle => 'Handle';
+
+  @override
+  String get publicProfileHandleHint =>
+      'Letters, digits and underscores, up to 30';
+
+  @override
+  String publicProfileAddress(String address) {
+    return 'Others find you as @$address';
+  }
+
+  @override
+  String get publicProfileHandleFixed =>
+      'Fixed since you published: others refer to it.';
+
+  @override
+  String get publicProfileBio => 'Bio';
+
+  @override
+  String get publicProfileLinks => 'Links';
+
+  @override
+  String get publicProfileLinksHint => 'Your site or other accounts, up to 4.';
+
+  @override
+  String get publicProfileLinkHint => 'https://…';
+
+  @override
+  String get publicProfileAddLink => 'Add link';
+
+  @override
+  String get publicProfileRemoveLink => 'Remove link';
+
+  @override
+  String get publicProfileSaved => 'Profile saved.';
+
+  @override
+  String get publicProfileRemove => 'Remove profile';
+
+  @override
+  String get publicProfileRemoveTitle => 'Remove your public profile?';
+
+  @override
+  String get publicProfileRemoveMessage =>
+      'Your handle becomes free for others to take.';
+
+  @override
+  String get publicProfileRemoved => 'Profile removed.';
+
+  @override
+  String get publicProfileFailed => 'Couldn\'t reach the instance. Try again.';
+
+  @override
+  String get publicProfileUnreachable =>
+      'Your public profile can\'t be reached right now.';
+
+  @override
+  String get publicProfileRetry => 'Retry';
+
+  @override
+  String get profileProblemHandleInvalid =>
+      'A handle is 1 to 30 letters (a to z), digits or underscores.';
+
+  @override
+  String get profileProblemHandleReserved =>
+      'This handle is kept for the instance.';
+
+  @override
+  String get profileProblemHandleTaken => 'Someone else has this handle.';
+
+  @override
+  String get profileProblemPublished =>
+      'You have published: your handle can\'t change, nor your profile go.';
+
+  @override
+  String get profileProblemDisplayNameTooLong => 'At most 30 characters.';
+
+  @override
+  String get profileProblemBioTooLong => 'At most 500 characters.';
+
+  @override
+  String get profileProblemTooManyLinks => 'At most 4 links.';
+
+  @override
+  String get profileProblemLinkInvalid =>
+      'Links start with https:// or http://.';
+
+  @override
+  String get profileProblemAvatarInvalid =>
+      'This picture can\'t be used: choose a PNG, JPEG, WebP or GIF image.';
+
+  @override
+  String get publicProfilePicture => 'Picture';
+
+  @override
+  String get publicProfileChoosePicture => 'Choose picture';
+
+  @override
+  String get publicProfileRemovePicture => 'Remove picture';
+
+  @override
+  String get publicProfilePictureType => 'Images';
+
+  @override
+  String get publicProfilePictureLater => 'Save your profile to add a picture.';
+
   @override
   String get settingsTitle => 'Settings';