feat: Form.TagPicker support

[bpavuk]

[ THIS IS INTENDED SOLELY FOR UI/UX REVIEW PURPOSES; PLEASE DON'T MERGE UNTIL DEBUG LOGGING IS REMOVED ]

closes #746, improves compatibility with Linear and Obsidian Raycast plugins.

a short overview:

• flake.nix, .gitignore, CMakeLists.txt - that's to use Qt Creator and/or qmlls on NixOS
• FormTagPicker.qml and TagPickerTag define UI
• parsing is spread thin across the codebase, but the most important parts are in tag-list.cpp and form-model.cpp
• fuzzy search implemented as a separate context property (is this how it's called?) because I can't think of a cleaner way to hand fuzzy search to C++

[bpavuk]

also, formatting in QML files is absolute hell. I did not take time to write .editorconfig for that