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>

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