feat(profile): profile pages on the account domain, and links verified by rel="me"

A profile now has a page on the web, at the root of the account domain
(https://halkyon.eu/@raisondetredev where the operator set ACCOUNT_DOMAIN,
else on the instance's web server), so the address belongs to the
identity rather than to Learn and stays when other services share it
(ADR 0011). The page shows what the app's profile sheet shows, in English
or German, without JavaScript or cookies, asks search engines not to list
it, marks every link rel="me" so other sites can verify links to it, and
sends a visitor who asked on another host to its own address. Caddy
forwards /@… to the web server, and serves nothing else on the account
domain; compose now passes ACCOUNT_DOMAIN on, which it never did.

A link is verified as Mastodon verifies one: its page links back to the
profile's with rel="me". profile.checkLinks fetches each link's page, at
most ten times an hour, over HTTPS on port 443 and to public addresses
only, connecting to the address it checked and verifying the certificate
for the host, with three redirects, 1 MiB and ten seconds at most, and
keeps what links back on the profile (verifiedLinks, a column of a table
no device syncs). A changed link loses its check, a new handle all of
them. The app shows the page's address to copy, checks a link saved by
itself and on Check links, and marks verified links in the form and in
others' profiles; a check answered after a newer save only adds its
checks.

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

halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -1881,6 +1881,9 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get profileSheetNotFound => 'Nobody on this instance has this handle.';
 
+  @override
+  String get profileSheetLinkVerified => 'Links back to this profile';
+
   @override
   String get profileSheetUnreachable =>
       'The profile couldn’t be loaded. Check your connection.';
@@ -1931,6 +1934,38 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get publicProfileAddLink => 'Add link';
 
+  @override
+  String publicProfilePage(String url) {
+    return 'Your profile page: $url';
+  }
+
+  @override
+  String get publicProfilePageCopy => 'Copy the address';
+
+  @override
+  String get publicProfilePageCopied => 'Address copied.';
+
+  @override
+  String get publicProfileVerifyHint =>
+      'A link gets a checkmark once its page links back to your profile page with rel=\"me\", as Mastodon checks links.';
+
+  @override
+  String get publicProfileCheckLinks => 'Check links';
+
+  @override
+  String get publicProfileLinkVerified => 'Links back to your profile';
+
+  @override
+  String publicProfileLinksChecked(int count, int verified) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'Checked: $verified of $count links link back to your profile.',
+      one: 'Checked: $verified of 1 link links back to your profile.',
+    );
+    return '$_temp0';
+  }
+
   @override
   String get publicProfileRemoveLink => 'Remove link';
 
@@ -2029,6 +2064,10 @@ class AppLocalizationsEn extends AppLocalizations {
   String get profileProblemAvatarInvalid =>
       'This picture can\'t be used: choose a PNG, JPEG, WebP or GIF image.';
 
+  @override
+  String get profileProblemTooManyLinkChecks =>
+      'The links were checked too often. Try again in an hour.';
+
   @override
   String get publicProfilePicture => 'Picture';