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/widgets/chain_install_confirm.dart

@@ -258,7 +258,13 @@ class ChainInstallConfirmDialog extends StatelessWidget {
           child: Text(l.buttonCancel),
         ),
         FilledButton(
-          onPressed: () => Navigator.pop(context, true),
+          // "blocked" means the hub WOULD refuse this install (policy
+          // demands signatures, no key material for this source) —
+          // offering an active Install button would contradict the
+          // statement right above it (doc-verifier finding).
+          onPressed: item.installVerification == 'blocked'
+              ? null
+              : () => Navigator.pop(context, true),
           child: Text(l.buttonInstall),
         ),
       ],