Surface plugin update availability on the plugins page

The plugin manager page already showed each installed plugin's version and
had an Update button, but nothing told users an update actually existed —
they had to guess. The store manager already compares the installed
manifest version against the registry's latest_version for its reinstall
decision; this surfaces that same signal in the UI.

- api_v3 /plugins/installed now returns `latest_version` (from the registry
  cache, no extra network call) and an `update_available` flag computed by a
  new semver-aware helper `_is_plugin_update_available`. A locally modified
  plugin whose version is ahead of the registry is not flagged.
- The installed-plugin card shows "vX.Y.Z available" next to the installed
  version, and the Update button becomes emphasized ("Update to vX.Y.Z" with
  a gentle pulse) when a newer version is published — mirroring the app's own
  update banner styling.
- Added tests for the helper and the endpoint fields.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012EW8yDbsk8EceDpq6Hjgqj

web_interface/blueprints/api_v3.py

@@ -120,6 +120,27 @@ def _get_plugin_version(plugin_id: str) -> str:
         logger.warning("[PluginVersion] Invalid JSON in manifest for %s at %s: %s", plugin_id, manifest_path, e)
     return ''
 
+def _is_plugin_update_available(installed_version: str, latest_version: str) -> bool:
+    """Return True when the registry's ``latest_version`` is strictly newer
+    than the installed version.
+
+    Uses PEP 440 / semver-aware comparison so a locally modified plugin whose
+    version is *ahead* of the published registry is not flagged as needing an
+    update. If either version string can't be parsed, falls back to a plain
+    inequality check (any difference is surfaced so the user can reconcile).
+    """
+    if not installed_version or not latest_version:
+        return False
+    if installed_version == latest_version:
+        return False
+    try:
+        from packaging.version import parse as _parse_version
+        return _parse_version(latest_version) > _parse_version(installed_version)
+    except Exception:
+        # Unparseable version string: we can't tell direction, so surface the
+        # mismatch rather than silently hiding a potential update.
+        return True
+
 def _ensure_cache_manager():
     """Ensure cache manager is initialized."""
     global cache_manager
@@ -2242,9 +2263,12 @@ def get_installed_plugins():
             if enabled is None:
                 enabled = plugin_instance.enabled if plugin_instance else True
 
-            # Verified from registry (no network call)
+            # Verified + latest published version from registry (no network call)
             store_info = api_v3.plugin_store_manager.get_registry_info(plugin_id)
             verified = store_info.get('verified', False) if store_info else False
+            latest_version = store_info.get('latest_version', '') if store_info else ''
+            installed_version = plugin_info.get('version', '')
+            update_available = _is_plugin_update_available(installed_version, latest_version)
 
             # Local git info (single subprocess on cache miss, zero on hit)
             plugin_path = Path(api_v3.plugin_manager.plugins_dir) / plugin_id
@@ -2291,6 +2315,8 @@ def get_installed_plugins():
                 'id': plugin_id,
                 'name': plugin_info.get('name', plugin_id),
                 'version': plugin_info.get('version', ''),
+                'latest_version': latest_version,
+                'update_available': update_available,
                 'author': plugin_info.get('author', 'Unknown'),
                 'category': plugin_info.get('category', 'General'),
                 'description': plugin_info.get('description', 'No description available'),