feat(gui): add origin abstraction layer for unified file operations

Implements Issue #9: Origin abstraction layer

This commit introduces a foundational abstraction for document origins,
enabling FreeCAD to work with different storage backends (local filesystem,
Silo PLM, future cloud services) through a unified interface.

## Core Components

### FileOrigin Abstract Base Class (FileOrigin.h/cpp)
- Defines interface for document origin handlers
- Identity methods: id(), name(), nickname(), icon(), type()
- Workflow characteristics: tracksExternally(), requiresAuthentication()
- Capability queries: supportsRevisions(), supportsBOM(), supportsPartNumbers()
- Connection state management with fastsignals notifications
- Document identity: documentIdentity() returns UUID, documentDisplayId() for display
- Property sync: syncProperties() for bidirectional database sync
- Core operations: newDocument(), openDocument(), saveDocument(), saveDocumentAs()
- Extended PLM operations: commitDocument(), pullDocument(), pushDocument(), etc.

### LocalFileOrigin Implementation
- Default origin for local filesystem documents
- ownsDocument(): Returns true if document has NO SiloItemId property
- Wraps existing FreeCAD file operations (App::GetApplication())

### OriginManager Singleton (OriginManager.h/cpp)
- Follows WorkbenchManager pattern (instance()/destruct())
- Manages registered FileOrigin instances
- Tracks current origin selection with persistence
- Provides document-to-origin resolution via findOwningOrigin()
- Emits signals: signalOriginRegistered, signalOriginUnregistered,
  signalCurrentOriginChanged
- Preferences stored at: User parameter:BaseApp/Preferences/General/Origin

### Python Bindings (FileOriginPython.h/cpp)
- Adapts Python objects to FileOrigin C++ interface
- Enables Silo addon to implement origins in Python
- Thread-safe with Base::PyGILStateLocker
- Static addOrigin()/removeOrigin() for registration

### Python API (ApplicationPy.cpp)
- FreeCADGui.addOrigin(obj) - Register Python origin
- FreeCADGui.removeOrigin(obj) - Unregister Python origin
- FreeCADGui.getOrigin(id) - Get origin info as dict
- FreeCADGui.listOrigins() - List all registered origin IDs
- FreeCADGui.activeOrigin() - Get current origin info
- FreeCADGui.setActiveOrigin(id) - Set active origin

## Design Decisions

1. **UUID Tracking**: Documents tracked by SiloItemId (immutable UUID),
   SiloPartNumber used for human-readable display only

2. **Ownership by Properties**: Origin ownership determined by document
   properties (SiloItemId), not file path location

3. **Local Storage Always**: All documents saved locally; origins change
   workflow and identity model, not storage location

4. **Property Syncing**: syncProperties() enables bidirectional sync of
   document metadata with database (Description, SourcingType, etc.)

## Files Added
- src/Gui/FileOrigin.h
- src/Gui/FileOrigin.cpp
- src/Gui/FileOriginPython.h
- src/Gui/FileOriginPython.cpp
- src/Gui/OriginManager.h
- src/Gui/OriginManager.cpp

## Files Modified
- src/Gui/CMakeLists.txt - Added new source files
- src/Gui/Application.cpp - Initialize/destruct OriginManager
- src/Gui/ApplicationPy.h - Added Python method declarations
- src/Gui/ApplicationPy.cpp - Added Python method implementations

Refs: #9

src/Gui/Application.cpp

@@ -137,6 +137,7 @@
 #include "Workbench.h"
 #include "WorkbenchManager.h"
 #include "WorkbenchManipulator.h"
+#include "OriginManager.h"
 #include "WidgetFactory.h"
 #include "3Dconnexion/navlib/NavlibInterface.h"
 #include "Inventor/SoFCPlacementIndicatorKit.h"
@@ -691,6 +692,9 @@ Application::Application(bool GUIenabled)
     if (GUIenabled) {
         createStandardOperations();
         MacroCommand::load();
+
+        // Initialize the origin manager (creates local origin by default)
+        OriginManager::instance();
     }
 }
 // clang-format on
@@ -702,6 +706,7 @@ Application::~Application()
     delete pNavlibInterface;
 #endif
     WorkbenchManager::destruct();
+    OriginManager::destruct();
     WorkbenchManipulator::removeAll();
     SelectionSingleton::destruct();
     Translator::destruct();