Gyh/dev

[ling0gyh]

Description:
Add post-processing evaluation and refinement stages to the image2drawio pipeline that detect
uncovered regions and rescue them as fallback image crops.

Changes:

metric_evaluator.py: Content-coverage scoring with three-channel bad-region detection (fine,
coarse, complex) plus banner channel for wide uncovered headers. Includes text-protection
filtering, NMS deduplication, union-coverage check against existing elements, and debug
visualization output.
refinement_processor.py: Fallback rescue for uncovered regions — crops bad regions from the
original image, filters out mostly-white regions, text-stroke regions, and regions already
covered by existing elements, then returns them as image elements for the final XML.
utils.py (sample_fill_stroke): Improved stroke color extraction — detects whether shapes have a
genuine dark border vs anti-aliased edges, avoiding false black stroke colors on colored shapes
(pink, teal, purple, etc.).
utils.py (extract_text_color): Fixed inconsistent bbox clipping bounds.
wf_paper2drawio_sam3.py: Integrated evaluate and refine nodes into the pipeline between
build_elements and render_xml.

This PR improves the editable PPT workflow by restructuring the frontend editable slide pipeline around a schema/canvas rendering
model, adding richer editable components, and supporting native client-side PPTX export.

Changes

• Added Canvas-based rendering for editable PPT slides, including structured layout nodes, visual specs, and layout IR capture.
• Added native client-side PPTX export for Canvas slides.
• Improved editable component support for text, callout/highlight, table, and image blocks.
• Added inline editing behavior for editable text/list fields directly on the slide preview.
• Added automatic deletion for empty editable text/callout content.
• Added support for deleting inserted images with the Delete key.
• Removed redundant editable PPT theme direction selection and simplified frontend theme controls.
• Improved placeholder behavior so missing text references no longer appear as visible error text.
• Moved selection/hover helper labels outside the slide canvas to avoid covering slide content.
• Adjusted HTML-to-PPTX scaling and export text ratio for better PowerPoint layout fidelity.
• Extended backend/frontend schema normalization and tests for the editable PPT flow.