Sketch origin marker: distinct display, properties UI, and active-sketch behavior

[trailcode]

Closes #177

Summary

• Draw the sketch Origin as a + inside a circle (configurable color in Settings → Sketch → Origin marker color); user-placed Add node markers stay red + without a circle.
• Show the origin annotation on the active sketch only, with a per-sketch Show origin marker toggle in Sketch properties.
• Add Position X/Y sliders (view-relative range, underlay-center pattern) and a Set floating input beside each slider (distance-edit behavior: Enter applies, Esc dismisses).
• [P] in the Sketch List makes that sketch current; while Sketch properties is open, the dialog tracks the current sketch.
• Origin edits do not push undo snapshots; show_origin_marker is persisted in sketch JSON.

Test plan

• Create two sketches; confirm origin circle appears only on the active sketch
• Toggle Show origin marker off/on in Sketch properties
• Change Origin marker color in Settings and confirm marker updates
• Drag X/Y sliders and use Set + Enter to type coordinates
• Click [P] on a non-current sketch — it becomes active and properties open for it
• With properties open, switch sketch via radio — properties panel updates
• Confirm origin still snaps when marker is hidden
• Save/reload project — origin position and show_origin_marker round-trip
• Run EzyCad_tests --gtest_filter=Sketch_test.*Origin*

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