feat(plugins): add register_dashboard_auth_provider hook on PluginContext

Phase 1, Task 1.3. Mirrors the existing register_image_gen_provider
pattern (plugins.py:531) — wrong-type or duplicate-name registrations
log at WARNING and silently return rather than raising, so a misbehaving
auth plugin cannot crash the host.

Deviation from plan: the plan's draft raised TypeError on non-provider
input; switched to silent-warn to match the established image_gen
convention. Test updated to match.

hermes_cli/plugins.py

@@ -553,6 +553,46 @@ class PluginContext:
             self.manifest.name, provider.name,
         )
 
+    # -- dashboard auth provider registration --------------------------------
+
+    def register_dashboard_auth_provider(self, provider) -> None:
+        """Register a dashboard authentication provider.
+
+        ``provider`` must be an instance of
+        :class:`hermes_cli.dashboard_auth.DashboardAuthProvider`. Used by
+        the dashboard OAuth auth gate, which engages when the dashboard
+        binds to a non-loopback host without ``--insecure``.
+
+        Misbehaving providers (wrong type, duplicate name) are logged at
+        WARNING and silently ignored — never raised — so a broken plugin
+        cannot crash the host. Same convention as
+        ``register_image_gen_provider``.
+        """
+        from hermes_cli.dashboard_auth import (
+            DashboardAuthProvider, register_provider,
+        )
+
+        if not isinstance(provider, DashboardAuthProvider):
+            logger.warning(
+                "Plugin '%s' tried to register a dashboard-auth provider "
+                "that does not inherit from DashboardAuthProvider. Ignoring.",
+                self.manifest.name,
+            )
+            return
+        try:
+            register_provider(provider)
+        except (TypeError, ValueError) as e:
+            logger.warning(
+                "Plugin '%s' failed to register dashboard-auth provider "
+                "%r: %s",
+                self.manifest.name, getattr(provider, "name", "?"), e,
+            )
+            return
+        logger.info(
+            "Plugin '%s' registered dashboard-auth provider: %s (%s)",
+            self.manifest.name, provider.name, provider.display_name,
+        )
+
     # -- video gen provider registration -------------------------------------
 
     def register_video_gen_provider(self, provider) -> None: