Fix slow XLSX import path and theme-color conditional formatting crash

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The first fix addresses the slow import path seen on larger or more complex
XLSX files. The importer now avoids unnecessary repeated XML work in the hot
path, which reduces import latency and makes workbook loading more stable in
the app.

The second fix addresses a workbook parsing crash caused by theme-based colors
in conditional formatting and DXF styles. Theme colors are now normalized once
at workbook load, and conditional-format font parsing uses the full workbook
style context. This prevents import failures on files like appointment-
schedule.xlsx that previously crashed during theme color resolution.