Naming cleanup: copy-pasted class names + stray operator categories

[Weisl]

Found via a cross-addon naming/wording consistency audit of the Simple family.

• Renamed EXPORTER_OT_open_preferences (copy-pasted from Simple Export, wrong domain prefix) -> CAM_MANAGER_OT_open_preferences; also fixed its label capitalization ("Open Addon Preferences" -> "Open Addon preferences") to match the other three addons' copies of this operator.
• Renamed the AddonPreferences class from the leftover CAM_MANAGER_OT_renaming_preferences (copy-pasted from Simple Renaming's VIEW3D_OT_renaming_preferences, and wrongly using an _OT_ operator-style name on a non-operator class) -> CamManagerAddonPreferences.
• Fixed stray cam.* keymap operator categories (cam.remove_hotkey, cam.key_selection_button) -> cam_manager.*, matching the addon's dominant category.
• Added a bpy.app.online_access check before the update-checker's network request, and declared [permissions].network in the manifest.

Changes are made locally (uncommitted) - not yet pushed.