fix(canvas): compensate print border in tool coordinate mapping

[marcinz606]

Problem

With finish.border_size > 0 (or paper aspect ≠ ORIGINAL) the preview canvas is padded with the print/mat border. The overlay stored the delivered content_rect but nothing used it: every tool click was normalized over the full padded frame while the pipeline expects content coords. Manual heal spots, dodge&burn lassos, mask-vertex edits and WB picks all landed offset by the border size (asymmetric with border_bottom_weight > 1). The reverse map had the same omission, so markers drew at the click point while the healed pixel landed elsewhere.

Cleared suspects: filed carrier and vignette are value-only (no geometry); rotation/fine-rot/flip/distortion/crop are already handled by uv_grid; stored config coords are raw-source-normalized, so no existing edits are affected.

Bonus bug found along the way: CPU mode padded the border-less crop/analysis preview too (main_window), misaligning the crop rect. GPU skips the layout pass there.

Fix

Single contract change in CanvasOverlay: every normalized coordinate crossing the overlay boundary is content-normalized, via one helper _content_view_rect() (short-circuits to _view_rect when there is no border).

• forward click map, _raw_to_screen, brush radius, drag clamps, mat-click gating, IR/wash raster draw targets
• get_pixel_rgb samples content-relative
• densitometer gets content_rect=None (was the only compensated path — would double-compensate)
• CPU crop/analysis preview no longer padded
• controller tool handlers and crop/analysis mapping untouched

Verification

• TDD: 9 new tests in tests/test_canvas_border_mapping.py; 7 failed pre-fix for the predicted reasons
• make all green: 2437 passed, lint + type clean
• Headless E2E (real GUI, offscreen, both engines): border 1.5 cm + bottom weight 1.6, real mouse click on a synthetic dust blob — stored stroke coords (0.3499, 0.4494) vs target (0.35, 0.45); healed-region centroid 0.5–0.7 px from the blob; the pre-fix mapping would have healed 23–25 px away. GPU and CPU: all checks pass.