Add owl sign-in page for the G2 run screen

First UI for the ops team: a local sign-in page where an owl sits on
the card, watches the password field as you type, shakes its head on
a wrong password and hops on the right one. Uses a test password for
now; where the real G2 password lives is still undecided.

Also adds PRODUCT.md (product context for UI work) and the page's
design brief under .impeccable/surfaces/.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -16,3 +16,6 @@ __pycache__/
 src/build/
 src/dist/
 build_log.txt
+
+# Impeccable review screenshots (regenerated each review)
+.impeccable/review/