fix(xlsx): render requested screenshot range

[Hanxcellent]

Summary

• pass xlsx screenshot cell ranges into the HTML renderer in both one-shot and resident paths
• extend the rendered sheet grid through the requested range corner before clipping
• reuse the existing xlsx range parser while preserving element clips such as /Sheet1/chart[1]

Fixes #246

Root cause

CaptureClipped resolves an xlsx range through its two corner cells. The HTML renderer only emitted cells through the sheet's computed data/drawing extent, so when the requested lower-right corner was outside that grid it did not exist in the DOM. The crop union then contained only the surviving upper-left corner and collapsed to that cell, excluding the chart and the rest of the requested area.

On current main, the default full-sheet screenshot already renders this repro chart. The remaining reproducible defect is the --range path when the requested range extends past the generated grid.

Validation

Repro workbook: populated cells in A1:B13 plus a floating chart starting at D1, then:

officecli view repro.xlsx screenshot \
  --range "Sheet1!A1:J20" \
  -o range.png

Observed result:

Version	PNG dimensions	Content
current main	120 x 40	only A1; chart omitted
this branch	1182 x 800	full A1:J20 range and chart

Additional checks:

# Existing element clip still works: 710 x 680
officecli view repro.xlsx screenshot --range '/Sheet1/chart[1]' -o chart.png

# Existing in-grid cell range is unchanged: 238 x 520
officecli view repro.xlsx screenshot --range 'Sheet1!A1:B13' -o cells.png

dotnet build src/officecli/officecli.csproj -c Release

The build succeeds with the pre-existing nullable warning in ExcelHandler.SheetShift.cs.

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