[PierreRaybaut]

Closes #69

Summary

Sigima and DataLab desktop define x0/y0 as the center of the first pixel (axis_to_physical(i) = x0 + i·dx), but the image viewers drew the first pixel from x0/y0 as its upper-left corner. Everything shown in physical coordinates (ROIs, analysis results, crosshair, stats rectangle, hover read-out) was therefore half a pixel off the pixels used by the computations. Example of the issue: a 4×4 image with x0 = y0 = 0 now spans [-0.5, 3.5] with pixel centers at 0…3.

• Shared axis helpers (src/utils/pixelCenters.ts): axisToFloatIndex / axisToPhysical mirror Sigima's axis_to_float_indices / axis_to_physical (extrapolation included), plus pixelEdges, imageExtent, imageCenter (Sigima xc/yc), pixelIndexAt and pixelRangeOverlapping, for uniform and non-uniform images. No other code converts between pixels and physical coordinates on its own anymore.
• Raster geometry (imageLod.ts, imageRaster.ts): ImageGeometry.x0/y0 are pixel centers; the bitmap starts at x0 + (i0 − ½)·dx. The placement fields are renamed x/y (Plotly layout-image names) to avoid the center/corner confusion, and a single rasterLayoutImage helper replaces three copies.
• Image viewer: extent, hover value (closest pixel center), cross-sections (index, abscissae, axis ranges) and stats. Cross-sections and stats now also work on non-uniform images; stats keep DataLab desktop's rule (every pixel the rectangle covers, even partly), now based on the real pixel edges.
• Spatial and thumbnail views, processing preview: extent and hover read-out likewise (non-uniform images included in the spatial hover).
• Default ROIs / erase area / ROI panel bounds: centered on the pixel grid of the image.
• ROI grid preview: the TypeScript replica had not followed Sigima 1.4 (grid laid out from x0, no pixel snapping). It now matches generate_image_grid_roi (grid over the outer pixel edges, whole-pixel cells, empty cells kept), and a zero translation is no longer read as the default.
• Decimated payloads (spatial view, processing preview source): x0' = x0 + (dx' − dx)/2 keeps the exact physical extent, so previews overlay the full-resolution image exactly.
• binSearchCell is removed (replaced by pixelIndexAt), along with its tests, which were ported to the new helpers.
• Changelog entry.

Out of scope: Distribute on grid / Reset positions keep the desktop behavior (they align first-pixel centers). The profile dialog already followed the convention (#70).

Validation

• vitest: 931 passed. New tests cover the helpers (issue example, calibrated and non-uniform images), LOD placement, ImagePlot (bitmap placement, hover value, stats, non-uniform cross-section), the spatial-view hover, and the ROI grid preview against reference values computed with Sigima. The new ImagePlot tests fail on develop.
• pytest: 530 passed (the decimation test now asserts the exact outer edges); ruff clean.
• tsc, eslint, prettier clean; npm run build within the bundle budget.
• Playwright (chromium, Sigima 1.4 wheel built from 3af8228): image-lod, multi-image-spatial, image-roi-drag, image-pan (9 passed).
• Browser check on the issue's 4×4 example: the bitmap spans [-0.5, 3.5], a ROI [-0.5, 1.5] lands on pixel edges, and hovering (0.6, 0.4) reads pixel (1, 0).

Note: small images are drawn smoothed by the browser when zoomed in (no image-rendering: pixelated on the main viewer's layout image, unlike the thumbnails). This existed before this PR and is not changed here.