fix memory leak and overflow issues

[sercanarga]

found and fixed a few critical bugs while reviewing the code:

memory leak in search
the case-insensitive search was corrupting the original text buffer by calling CharLowerBuffW directly on it. now we create a separate copy for comparison, keeping the original safe.

overflow protection in replace all
when replacing text in large files, the buffer size calculation could overflow. added a check to prevent crashes when the result would be too large.

better default filename
changed the save dialog default from *.txt (which is a wildcard) to Untitled.txt -which makes more sense-.

tested all fixes with msvc, no warnings or issues found