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

[trailcode]

Summary

Enable ImGui docking (native multi-viewport + WASM in-canvas), fix several Add node tool bugs around distance input / undo / redo, fix circle/arc undo-redo regressions, clear undo history on New Project, fix chamfer/fillet view crash after extrude + view-cube navigation, and refresh bundled default settings.

ImGui docking

• Upgrade vendored Dear ImGui to v1.92.7-docking (IMGUI_HAS_DOCK).
• Native: dockable/tabbed panels inside the main window; panels can detach into separate OS windows (multi-viewport).
• WASM: in-canvas docking only (single browser canvas).
• Persist dock layout in imgui_ini inside ezycad_settings.json; older installs without a [Docking] section get a default layout on first launch.
• Settings -> UI: separate Dark / Light theme ImGui controls (transparency, rounding, borders, padding, spacing). Legacy imgui_rounding_* keys migrate to both themes on load.

Add node tool fixes

1. Escape during distance input — pressing Escape while typing a distance no longer commits the node; distance/angle popups cancel instead of finalize.
2. Undo after anchor + distance or second click — Sketch_op_recorder now opens before node creation on rubber-band placement paths so undo deltas are recorded.
3. Redo — Sketch_delta::apply_forward_ restores curr_node_pts (revive tombstoned nodes and re-run interior splits).
4. Length dimension tool + Origin — try_pick_existing_node no longer hard-excludes the sketch Origin (still respects origin_snap_enabled).

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 recorded for arc creation and then split arcs at those interior bulge points. Bulge nodes were also revived as permanent via get_node_exact(pt, true).

Fix (in PR #180): restore_curr_node_at_pt_() skips interior splits for arc bulge points and does not promote arc-defining nodes (pt_a / pt_b / pt_c from curr_arc_edges) to permanent on redo. Add-node redo behavior is unchanged.

Verified: circle undo/redo keeps 2 edges and the same permanent node count; Arc Segment undo/redo keeps 1 edge.

View / chamfer fix

Repro: new project, rotate view, click view cube Top (zoom to origin), zoom out, add square, extrude, chamfer, then change view — camera jumped to empty space and the app crashed in FlushViewEvents (Standard_ProgramError).

Root cause: chamfer/fillet replaced the picked solid without clearing AIS selection/highlight on the removed shape; switching from sketch orthographic (extrude) to shape perspective (chamfer) after shapes were already redisplayed corrupted camera zoom.

Fix (in PR #180):

• replace_picked_shape_() for chamfer/fillet: clear hover/selection, remove old shape, Display replacement.
• apply_camera_projection() at start of on_mode() (before shape visibility/selection updates).
• Preserve eye-center distance and view scale when toggling ortho/perspective.
• Defensive try/catch in flush_view_events().

Verified: chamfer no longer crashes; view cube Top orients correctly; solid stays visible after chamfer.

Other fixes

• New Project clears undo and redo stacks instead of pushing a snapshot of the previous document.
• Distance edit popup follows the cursor when reopened (Tab) instead of staying at the first-open position.
• Bundled defaults (res/ezycad_settings.json) updated to current preferred settings (layout, theme, grid off, dimension style, snap colors, etc.).

Tests

• AddNode_distance_enter_undo
• AddNode_anchor_then_click_undo (undo + redo)
• DimensionTool_picks_sketch_origin
• NewFile_clears_undo_stacks
• Undo_single_arc_via_recorder (GUI Arc Segment 3-click path)
• Undo_circle_via_recorder (GUI Circle center + radius path; checks edge and permanent-node counts after redo)

Documentation

• CHANGELOG.md (UI layout section)
• docs/usage-settings.md (docking, settings storage, UI theme keys)
• src/doc/gui.md (input routing, dock space)

[trailcode]

View/chamfer fix pushed in c3f3256 on \Trailcode/imgui-docking:
eplace_picked_shape_(), projection applied before shape redisplay in \on_mode(), and scale/distance preserved on ortho/perspective switch. Manual repro verified — no crash, view cube Top behaves correctly.