Check plugin dependencies before loading, warn instead of silent console error

manifest.json already had an unused "requires" field (motion_capture's
manifest lists opencv-python>=4.8.0) — PluginManifest never parsed it,
so a missing dependency just crashed the import inside _load_plugin(),
caught by the broad except and only ever printed to the console.

Adds PluginManifest.requires, PluginManager.missing_requirements()/
get_missing_dependencies(), and an early check in _load_plugin() that
skips the risky import entirely when a requirement is missing.
main_window.plugin_enable() now checks this before calling
PluginManager.enable() and shows a QMessageBox with the missing
packages and a pip install command instead of failing silently.

Checks by distribution name via importlib.metadata (what pip installed
it as), not import name — those differ for packages like opencv-python
(imports as cv2) or pyserial (imports as serial), so importlib.util.
find_spec() would give false negatives.

Also fixes a button-state bug this surfaces: SettingsWindow's plugin
toggle optimistically flipped to "Disable" the instant Enable was
clicked, before knowing whether enabling actually succeeds — pre-existing,
but now trivially reproducible (any plugin missing a dependency). Enable
no longer flips the button immediately; main_window calls the new
sync_plugin_button() once the real outcome is known.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

3 files changed, 66 insertions, 6 deletions

diff --git a/plugins/plugin_manager.py b/plugins/plugin_manager.py
index 36ae71a..5cefc2e 100644
--- a/plugins/plugin_manager.py
+++ b/plugins/plugin_manager.py
@@ -10,12 +10,14 @@ per-plugin state separately via get_save_state / apply_save_state.
 
 from __future__ import annotations
 
+import importlib.metadata
 import importlib.util
 import json
 import os
+import re
 import sys
 import traceback
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import Dict, List, Optional
 
 from plugins.base_plugin import LabPlugin, PluginContext
@@ -35,9 +37,35 @@ class PluginManifest:
     description: str = ""
     author:      str = ""
     entry_point: str = "plugin.Plugin"   # "module.ClassName" relative to plugin dir
+    requires:    List[str] = field(default_factory=list)  # pip-style reqs, e.g. "opencv-python>=4.8.0"
     plugin_dir:  str = ""
 
 
+def _dist_name(requirement: str) -> str:
+    """Extract the distribution name from a requirement string, e.g.
+    "opencv-python>=4.8.0" -> "opencv-python"."""
+    return re.split(r"[<>=!~\[; ]", requirement.strip(), maxsplit=1)[0]
+
+
+def missing_requirements(requires: List[str]) -> List[str]:
+    """Return the subset of `requires` whose distribution isn't installed.
+
+    Checked by distribution name via importlib.metadata (matches what pip
+    installed it as), not by import name — those differ for packages like
+    opencv-python (imports as cv2) or pyserial (imports as serial).
+    """
+    missing = []
+    for req in requires:
+        name = _dist_name(req)
+        if not name:
+            continue
+        try:
+            importlib.metadata.version(name)
+        except importlib.metadata.PackageNotFoundError:
+            missing.append(req)
+    return missing
+
+
 # ── Manager ───────────────────────────────────────────────────────────────────
 
 class PluginManager:
@@ -94,6 +122,7 @@ class PluginManager:
                     description = data.get("description", ""),
                     author      = data.get("author", ""),
                     entry_point = data.get("entry_point", "plugin.Plugin"),
+                    requires    = data.get("requires", []),
                     plugin_dir  = plugin_dir,
                 )
                 self._manifests[m.plugin_id] = m
@@ -146,6 +175,11 @@ class PluginManager:
             print(f"[PluginManager] No manifest for '{plugin_id}'")
             return None
 
+        missing = missing_requirements(manifest.requires)
+        if missing:
+            print(f"[Plugin] '{plugin_id}' missing dependencies: {', '.join(missing)}")
+            return None
+
         module_name, class_name = manifest.entry_point.rsplit(".", 1)
         module_file = os.path.join(
             manifest.plugin_dir, *module_name.split("/")
@@ -218,6 +252,10 @@ class PluginManager:
     def get_manifests(self) -> List[PluginManifest]:
         return list(self._manifests.values())
 
+    def get_missing_dependencies(self, plugin_id: str) -> List[str]:
+        manifest = self._manifests.get(plugin_id)
+        return missing_requirements(manifest.requires) if manifest else []
+
     def get_loaded(self) -> List[LabPlugin]:
         return list(self._loaded.values())
 
diff --git a/ui/main_window.py b/ui/main_window.py
index d0e0ac4..95cdb84 100644
--- a/ui/main_window.py
+++ b/ui/main_window.py
@@ -296,10 +296,22 @@ class MainWindow(QMainWindow):
 
     def plugin_enable(self, plugin_id: str):
         """Called by SettingsWindow when user enables a plugin."""
-        ctx = self._make_plugin_context()
-        plugin = self._plugin_mgr.enable(plugin_id, ctx)
-        if plugin:
-            self._install_plugin(plugin)
+        missing = self._plugin_mgr.get_missing_dependencies(plugin_id)
+        if missing:
+            from PyQt6.QtWidgets import QMessageBox
+            QMessageBox.warning(
+                self, "Missing Plugin Dependencies",
+                f"Can't enable this plugin — missing Python packages:\n\n"
+                f"  {', '.join(missing)}\n\n"
+                f"Install with:\n  pip install {' '.join(missing)}"
+            )
+        else:
+            ctx = self._make_plugin_context()
+            plugin = self._plugin_mgr.enable(plugin_id, ctx)
+            if plugin:
+                self._install_plugin(plugin)
+        if self._win_settings:
+            self._win_settings.sync_plugin_button(plugin_id)
 
     def plugin_disable(self, plugin_id: str):
         """Called by SettingsWindow when user disables a plugin."""
diff --git a/ui/windows/settings_window.py b/ui/windows/settings_window.py
index 1630efa..553a3a7 100644
--- a/ui/windows/settings_window.py
+++ b/ui/windows/settings_window.py
@@ -50,6 +50,7 @@ class SettingsWindow(QWidget):
         self.registry       = registry
         self.engine         = engine
         self._plugin_mgr    = plugin_manager
+        self._plugin_buttons: dict = {}   # plugin_id -> QPushButton
         self.cfg            = dict(self._defaults)
         if current:
             self.cfg.update(current)
@@ -233,6 +234,7 @@ class SettingsWindow(QWidget):
         toggle.clicked.connect(
             lambda _, pid=manifest.plugin_id, btn=toggle: self._toggle_plugin(pid, btn)
         )
+        self._plugin_buttons[manifest.plugin_id] = toggle
         hdr.addWidget(toggle)
         cl.addLayout(hdr)
 
@@ -261,8 +263,16 @@ class SettingsWindow(QWidget):
             self.plugin_disable_requested.emit(plugin_id)
             btn.setText("Enable")
         else:
+            # Don't flip to "Disable" yet — enabling can fail (missing
+            # dependencies, bad plugin code). main_window confirms the
+            # real outcome via sync_plugin_button() once enable() returns.
             self.plugin_enable_requested.emit(plugin_id)
-            btn.setText("Disable")
+
+    def sync_plugin_button(self, plugin_id: str):
+        """Refresh one plugin's toggle button to match its actual enabled state."""
+        btn = self._plugin_buttons.get(plugin_id)
+        if btn is not None and self._plugin_mgr is not None:
+            btn.setText("Disable" if self._plugin_mgr.is_enabled(plugin_id) else "Enable")
 
     # ── Actions ───────────────────────────────────────────────────────────