feat: unified DataDirectory model for preset data access

[hikaps]

Summary

• Replaces 5 scattered data directory mechanisms (sharedDirectories, needsDataAcl, needsConfigCopy, gameDirectories, setupLauncherAccess/setupSharedDirectories) with a single unified DataDirectory model
• Each data directory entry has a path + mode (copy | overlay | acl), fully configurable per preset
• Launcher-specific config managers (Steam, Heroic) provide prepare/finalize hooks (prepareDataDir() / finalizeDataDir()) for launcher-specific setup around generic operations
• Adds OverlayFS mounting (SetupOverlayMount) and directory copying (CopyDirectoryToUser) to the D-Bus helper service
• Custom presets work out of the box with pure generic operations (null configManager → hooks skipped)

Architecture

DataDirectory { path: "/home/user/.steam", mode: "overlay" }
        │
        ▼
setupDataDirectories() in SessionRunner
  ├── For each directory:
  │     ├── configManager->prepareDataDir(path, mode, user)  // Steam: loads library+ACLs
  │     ├── dispatch by mode:
  │     │     ├── "acl"    → SetPathAclWithParents
  │     │     ├── "copy"   → CopyDirectoryToUser
  │     │     └── "overlay"→ SetupOverlayMount (upperdir per user)
  │     └── configManager->finalizeDataDir(path, mode, user) // Steam: copies manifests+VDF

Test plan

• cmake --build build passes clean
• ctest --test-dir build --output-on-failure — 12/14 pass (2 pre-existing: flatpak test on build host, D-Bus helper version test)
• test_presetmanager — 230 lines of new tests for DataDirectory model, serialization, defaults
• test_sessionrunner — updated for new unified dispatcher API
• test_gamescopeinstance — updated for launcherId changes
• Grep confirmed zero references to removed symbols across codebase
• Manual: verify overlay mount creates correct upperdir structure
• Manual: verify QML mode selector in EditPresetDialog

Breaking changes

• sharedDirectories config key replaced by dataDirectories (legacy migration reads old key as fallback, converts to acl mode)
• needsDataAcl, needsConfigCopy, gameDirectories removed from LauncherInfo
• setupLauncherAccess() and setupSharedDirectories() removed from SessionRunner