Add option to install ROMs flat

[matthew-pye]

• Add option to install ROM flat instead of installing ROMs in a folder with the same name as the game

[gantoine]

@greptileai

[greptile-apps]

Greptile Summary

Adds a per-emulator option for storing ROM files directly in the configured destination.

• Persists and exposes the new Install Flat mapping setting.
• Uses the destination root for flat imports and downloads.
• Adjusts imported ROM and installation paths for flat storage.

Confidence Score: 2/5

This PR is not safe to merge because routine failure cleanup or uninstall can delete every ROM in a flat destination, and archive installs can attach sibling files to the wrong game.

The new flat mode replaces the game-specific install directory with a shared mapping root while downstream cleanup, uninstall, and ROM discovery continue treating that path as exclusively owned by one game.

Files Needing Attention: Games/RomMInstallController.cs and Games/RomMImport.cs

Important Files Changed

Filename	Overview
Games/RomMInstallController.cs	Routes flat installations through a shared root that existing discovery and recursive cleanup logic assumes is game-owned.
Games/RomMImport.cs	Records the shared destination root as each flat-installed game's InstallDirectory, exposing destructive uninstall behavior.
Settings/EmulatorMapping.cs	Adds a serializable, property-change-aware InstallFlat setting.
Settings/SettingsView.xaml	Adds the per-mapping Install Flat checkbox and explanatory tooltip.

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
Games/RomMInstallController.cs:51-52
**Shared root is recursively deleted**

When a flat installation fails, is cancelled, or is later uninstalled, the shared destination is treated as the game's private install directory and recursively deleted, causing every sibling ROM in that destination to be removed.

### Issue 2 of 2
Games/RomMInstallController.cs:51-52
**ROM discovery includes sibling games**

When an archive is installed into a flat destination containing other supported ROM files, `BuildRoms` recursively scans the shared directory and adds those sibling files to the newly installed game's launch targets.

_{Reviews (1): Last reviewed commit: "Add install flat option" | Re-trigger Greptile}

[gantoine]

nice!