ImGui docking, add-node undo/redo fixes, and settings defaults

[trailcode]

Closes #179

Summary

• Upgrade Dear ImGui to v1.92.7-docking and enable dockable panels (native multi-viewport; WASM in-canvas only).
• Add Settings -> UI dark/light theme controls; persist dock layout in imgui_ini.
• Fix Add node distance-input Escape handling, undo/redo for rubber-band placement, and length-dimension picking of the sketch Origin.
• Fix circle and arc undo/redo so redo restores the same edge count and permanent nodes as the initial create.
• New Project now clears undo/redo stacks.
• Fix chamfer/fillet view crash and zoom corruption after sketch extrude + view-cube navigation.
• Refresh bundled res/ezycad_settings.json defaults (layout, theme, grid, dimensions, snap colors).

Add node fixes

Bug	Fix
Escape during distance input still added node	hide_dist_edit(false) / hide_angle_edit(false) on Escape; route Escape to on_key while dist edit is focused
Undo empty after anchor + distance/click	Open Sketch_op_recorder before add_new_node / get_node_exact in rubber-band commit paths
Redo did not restore placed nodes	restore_curr_node_at_pt_() in Sketch_delta::apply_forward_
Length dimension tool ignored Origin	Remove hard origin exclusion from try_pick_existing_node

Circle / arc undo-redo fix

Repro: create a circle (center + radius click), undo, redo — debug pane showed 4 edges and 3 extra permanent nodes instead of the original 2 edges.

Root cause: on redo, restore_curr_node_at_pt_() revived stencil/bulge nodes from curr_arc_edges and then called split_arcs_at_node_if_interior, splitting each semicircle in half. Bulge nodes were also revived with get_node_exact(pt, true), promoting them to permanent.

Fix: in restore_curr_node_at_pt_(), treat nodes matching arc-defining points (pt_a / pt_b / pt_c in curr_arc_edges) specially:

• Bulge (pt_b): revive without making permanent; skip interior splits.
• Endpoints (pt_a / pt_c): revive without making permanent (split is already a no-op at endpoints).
• Add-node placements (not arc-defining): unchanged — revive as permanent and re-run interior splits.

View / chamfer fix

Repro: new project, rotate view, view cube Top (zoom to origin), zoom out, draw square, extrude, chamfer, then rotate/zoom — view jumped to empty space and FlushViewEvents crashed with Standard_ProgramError.

Problem	Fix
Stale selection/highlight on removed solid after chamfer/fillet	replace_picked_shape_(): clear hover/selection, Unhilight, remove old shape, Display replacement
Orthographic sketch extrude -> perspective chamfer corrupted zoom	apply_camera_projection() runs at start of on_mode() (before shape show/redisplay)
Projection switch reset camera scale/distance	Preserve eye-center distance and view scale when toggling ortho/perspective
Residual OCCT failures during navigation	Defensive try/catch in flush_view_events() clears selection/hover and aborts view animation

Settings storage (for reference)

Platform	Path
Windows (live)	%APPDATA%\EzyCad\ezycad_settings.json
Bundled defaults	res/ezycad_settings.json
Linux	~/.config/EzyCad/ezycad_settings.json
macOS	~/Library/Application Support/EzyCad/ezycad_settings.json

Test plan

• EzyCad_tests.exe --gtest_filter=Sketch_test.AddNode_*:Sketch_test.NewFile_clears_undo_stacks:Sketch_test.DimensionTool_picks_sketch_origin
• EzyCad_tests.exe --gtest_filter=Sketch_test.Undo_single_arc_via_recorder:Sketch_test.Undo_circle_via_recorder
• Manual: dock/tab/split panels; detach panel to OS window (native); layout persists across restart
• Manual: Add node from Origin -> Tab -> distance -> Enter -> Ctrl+Z -> Ctrl+Y
• Manual: Add node from Origin -> Tab -> distance -> Escape (no node added)
• Manual: File -> New -> debug pane shows Undo: 0 | Redo: 0
• Manual: Circle tool (center + radius) -> Ctrl+Z -> Ctrl+Y — still 2 edges, no extra permanent nodes
• Manual: Arc Segment tool (3 clicks) -> Ctrl+Z -> Ctrl+Y — still 1 edge
• Manual: Settings -> Defaults restores bundled layout/theme
• Manual: new project -> rotate -> view cube Top -> zoom out -> square -> extrude -> chamfer -> rotate/zoom (no crash, solid stays visible)
• Manual: view cube Top orients to top view (not bottom)
• WASM smoke: docking inside canvas; no multi-viewport

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