Fix Replace selection handling and no-match feedback

[HaruLerrz]

This fixes two issues in the Replace dialog.

Changes:

• Uses RichEdit's EM_GETSELTEXT to read the current selection before replacing it, instead of using RichEdit selection positions as indices into a separate std::wstring.
• Shows the existing "Cannot find" message when Replace All finds zero matches.

The Find implementation itself is unchanged by this PR.

Tested locally with:

• manually selecting a matching string and replacing it successfully
• Replace All with multiple matches
• Replace All with zero matches showing the existing "Cannot find" message

Subsequent Find behavior after a replacement is unchanged and is covered separately by #38.