feat: a launch screen with the kingfisher and "Halkyon" over "Learn"

While the app opens (the instance, the session, the person's data), it
showed a spinner, in three ways. It now shows the kingfisher, and beneath
it "Halkyon" over "Learn", cobalt and copper as the logo has them, in the
app's own Noto Sans (your call, with JetBrains Mono to compare: the
logo's ITC Avant Garde isn't free to ship), and a quiet line of progress.

AGENTS.md now has the rule on text fields' limits, busy sheets and the
launch screen.

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_en.arb

@@ -2,6 +2,8 @@
   "@@locale": "en",
 
   "appTitle": "Halkyon Learn",
+  "launchBrand": "Halkyon",
+  "launchProduct": "Learn",
   "@appTitle": {"description": "The app's name, e.g. in the window title and the rail's logo."},
 
   "shellTabHome": "Home",