fix(xlsx): make import mark document modified to save xlsx properly

[asomethings]

Import changes cells in memory, but it never sets Modified = true.

In v1.0.143 (cd4befc) Dispose() learned to throw away all in-memory changes when Modified is false. So officecli import prints a success message like Imported 3 rows x 2 cols, but nothing is saved to the file. The same bug was already fixed for Move/Swap/CopyFrom in c84340d. But Import was missed.

Fix: set Modified = true right before SaveWorksheet(worksheet), so Dispose() saves the imported cells. An empty import returns before this line, so it stays a no-op.

Validation (before → after):

officecli create book.xlsx
printf 'name,value\nalpha,10\nbeta,20\n' > data.csv
officecli import book.xlsx /Sheet1 data.csv --header
officecli close book.xlsx
officecli get book.xlsx /Sheet1/A1

• Before (v1.0.143): import prints success, but /Sheet1/A1 is (empty) ✗ — nothing on disk
• After: /Sheet1/A1 (cell) "name" type=String ✓ and /Sheet1/B3 (cell) "20" type=Number ✓

Also checked the batch path — an import item with no set after it, which was broken the same way:

officecli create book2.xlsx
printf '[{"command":"import","parent":"/Sheet1","text":"x,y\n1,2\n3,4"}]' | officecli batch book2.xlsx
officecli close book2.xlsx
# the imported values are now in xl/worksheets/sheet1.xml on disk