fix(store,doctor): doc-verifier findings on the trust surfaces

The doc-verifier pass over the new trust/exposure surfaces came back
PASS with five improvements, all applied:
- the store policy notice gains a 'Learn more' into the security doc
  (the notice named security.require_signatures but not where it
  lives)
- 'blocked' disables the trust gate's install button — an active
  button contradicted the 'install would be refused' statement right
  above it (guard test added)
- the unknown-exposure tooltip now says what the operator can do
  (check where the name resolves)
- dead l10n key verifPillUnverified removed (unverified is the
  page-level notice, never a card pill)
- stale header comment in install_verification.dart corrected

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/l10n/app_localizations_en.dart

@@ -2171,7 +2171,7 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get svcExposureUnknownHint =>
-      'Hostname instead of an IP address — the hub deliberately does not resolve names to classify.';
+      'Hostname instead of an IP address — the hub deliberately does not resolve names to classify. Check yourself where the name resolves.';
 
   @override
   String get doctorLinkStore => 'Open the store';
@@ -2906,9 +2906,6 @@ class AppLocalizationsEn extends AppLocalizations {
   String get storePolicyUnverifiedNotice =>
       'Signature enforcement is switched off in the hub policy — installs are not cryptographically verified. The install dialog shows the per-module status; enable security.require_signatures for verified installs.';
 
-  @override
-  String get verifPillUnverified => 'unsigned';
-
   @override
   String get verifPillBlocked => 'blocked';