feat: embed cell <img> as floating images in exported excel

[sibinc]

Problem

Images placed inside table cells (<td>/<th>) — college logos, header/banner images, signatures, seals, student photos, QR codes — were silently dropped from the exported Excel. htmldecode() strips every tag (including <img>), so the cell became empty text and no image was ever embedded. This affects 100+ report templates that render an <img> into a header cell and export via TableToExcel.convert(...).

Fix

Add image support in the library itself (one place), so no template or caller changes are needed:

• src/parser.js — parseDomToTable now receives the workbook. For each cell, addCellImages() finds every <img>, reads the already-loaded DOM image through a <canvas> → PNG base64, and embeds it with wb.addImage() + ws.addImage() anchored (oneCell) to the cell's top-left, sized from the img's width/height. Cell text alongside the image is still exported. Wrapped in try/catch — unloaded or cross-origin-tainted images are skipped so a single bad image never breaks the export.
• src/tableToExcel.js — threads the workbook into parseDomToTable.
• convert() stays synchronous (canvas reads are sync for loaded, same-origin images), so existing callers are untouched.

Tests

• Updated existing parser test call sites to the new signature.
• Added an image-cell sample + test (graceful-degradation path).
• 30/30 passing.

Notes for reviewers

• Images are read from the rendered DOM, so the report must be visible before export.
• Because a canvas is used, images must be same-origin or served with CORS headers; otherwise that single image is skipped (rest of the sheet still exports).
• dist/ bundle is rebuilt in this PR. Deploying to consumers (e.g. examcontroller-ui) requires publishing a new package version and bumping it there.