Curate and apply safe upstream Carnac PRs

[Copilot]

This PR reviews the open upstream Code52/carnac PR backlog and ports only the changes that are still reasonable, low-risk, and compatible with this fork. Broad stale behavior changes were intentionally left out; the applied set focuses on display positioning, keymap coverage, and accurate setup docs.

• Applied upstream changes

  • Ported the vertical multi-display placement fix from upstream PR Add support for vertical multiple display setup. Code52/carnac#238
  • Added Kdenlive keymap support from upstream PR Adding keymaps for Kdenlive Code52/carnac#303
  • Folded in the safe documentation improvements from upstream README PRs (Update chocolatey things Code52/carnac#329 / docs: Update setup docs Code52/carnac#352)

• Display positioning

  • Added PopupSettings.Top alongside the existing Left coordinate
  • Updated screen placement to persist both Left and Top for the selected monitor
  • Updated KeyShowView to react to Top changes, so overlays position correctly on vertically arranged displays
  • Removed preview scaling from ScreenManager.Top, so saved coordinates use actual monitor origin values

• Keymap support

  • Added src/Carnac.Logic/Keymaps/kdenlive.yml
  • Registered the new keymap in Carnac.Logic.csproj
  • Normalized shortcut entries to the syntax supported by the current legacy parser, avoiding dead or non-parsable mappings

• Docs cleanup

  • Updated installation instructions to include WinGet
  • Replaced deprecated cinst usage with choco install carnac
  • Corrected contributor setup guidance to reference the existing PowerShell build entrypoint in this fork

• Focused coverage

  • Added a view-model test to verify that selecting a screen copies both horizontal and vertical origin values into settings

Settings.Left = SelectedScreen.Left;
Settings.Top = SelectedScreen.Top;

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