chat(build): fix broken installer on macOS

[cebtenzzre]

• The first commit fixes a regression in repo: organize sources, headers, and deps into subdirectories #2917 that caused default.metallib to not be packaged in the GPT4All installer.
• The second commit fixes a problem with [Feature Branch] - major new UI redesign #2396 that caused two copies of Nomic Embed to be shipped on macOS - one in /Applications/gpt4all/bin and one in /Applications/gpt4all/bin/GPT4All.app/Contents/Resources. Only the second one is used. This saves 262MiB of disk space.
• The third commit partially reverts repo: organize sources, headers, and deps into subdirectories #2917. For some reason, the structure with qt_add_executable in src/CMakeLists.txt causes the packaged version of GPT4All to fail to find the bundled Qt on macOS. So src/qml/ is moved back to qml/, src/main.qml is moved back to main.qml, and src/CMakeLists.txt is eliminated. I have yet to find a way to build a fully working version of GPT4All with the qml/ folder inside of src/.

With these changes, the offline macOS installer I built locally is smaller and produces a working copy of GPT4All.

[cebtenzzre]

CI no longer displays the warning about code signing, which must have been related to the third bullet point in the OP.