feat(profile): a handle in a studyset opens its person's profile

The summary's first block now reads as a studyset's card: its secondary
title, its title, then by @handle; the blocks' second heading is
Material. A licence holder written as a handle of the instance
(@raisondetredev) reads © raisondetredev, the © standing for the @,
and is a link, apart from a name from elsewhere such as Tae Kim's; it
and the author open their public profile in a sheet: avatar, name,
address, bio and links, as the instance's profiles.byHandle gives them.

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_en.arb

@@ -531,6 +531,9 @@
   "profileSwitchStay": "Stay anonymous",
   "profileSwitchConfirm": "Switch",
   "publicProfileTitle": "Public profile",
+  "profileSheetTitle": "Profile",
+  "profileSheetNotFound": "Nobody on this instance has this handle.",
+  "profileSheetUnreachable": "The profile couldn’t be loaded. Check your connection.",
   "publicProfileIntro": "How others see you once you publish: the handle they find you by, your name, a few words and links. It saves as you write, once you have chosen your handle.",
   "publicProfileAnonymousHint": "Sign in with GitHub to have a public profile and publish.",
   "publicProfileDisplayName": "Display name",
@@ -1904,7 +1907,7 @@
   "@exploreBy": {"placeholders": {"author": {"type": "String"}}},
   "exploreRelease": "v{version} · {date}",
   "@exploreRelease": {"placeholders": {"version": {"type": "int"}, "date": {"type": "String"}}},
-  "exploreHolds": "What it holds",
+  "exploreMaterial": "Material",
   "exploreLatestRelease": "Latest release: v{version} · {date}",
   "@exploreLatestRelease": {"placeholders": {"version": {"type": "int"}, "date": {"type": "String"}}},
   "exploreAddToLibrary": "Add to library",