fix(media): keep dotted input stems intact in folder output names

[kevin9327]

folder_output_path decided whether the output pattern had an extension by looking at the substituted name. If the input stem contains a dot, that stem looks like an extension:

• clip.v2.mkv with pattern {original}_restored gave clip.v2_restored, with no .mkv. Its "suffix" is .v2_restored, so the input extension was never added and the encoder got a path with no real container extension.
• a.b.png with pattern {original}_restored gave a.png. with_suffix replaced .b_restored, so the pattern text was lost, and a.b.png and a.c.png both mapped to the same output.

This affects the CLI folder mode, image restore, and the GUI queue/processor, which all call this helper.

Fix: take the extension from the pattern itself (with {original} removed) rather than from the result. Patterns with and without an extension behave as before for undotted names.

Tests: I added three cases to TestFolderOutputPath in tests/test_media_files.py. The two dotted-stem cases fail on main and pass with the fix, and all 14 tests in the file pass.

AI-assistance note: I found and wrote this change with help from an AI coding assistant. I reviewed it and ran the tests myself.