Installer: switch to versioned folder layout with Current junction

Replaces flat {app}\ deployment with versioned Versions\<ver>\ + Current junction.
Eliminates PendingUpgrade staging flow — new version goes to new folder, junction
swaps atomically, mounts continue running from old version until unmounted.

Changes:
- [Files]: Deploy to {app}\Versions\{version}\ instead of {app}\
- [Registry]: PATH points to {app}\Current (junction) instead of {app}\
- [Dirs]: ProgramData under versioned folder
- [Code]: Add CreateOrUpdateCurrentJunction() — creates/updates junction post-install
- [Code]: Add GarbageCollectOldVersions() — keeps 1 most recent old version, deletes older
  (skips versions with running mounts detected via Get-Process gvfs.mount | .Path)
- [Code]: Remove KeepMountsRunning variable, IsNormalInstall/IsStagingInstall checks
- [Code]: Remove StagingUpdateService, ShowMountChoiceDialog (no longer needed)
- [Code]: Simplify PrepareToInstall — no mount detection, no staging, just stop service
- [Code]: Update InstallGVFSService to reference {app}\Current\GVFS.Service.exe
- [Code]: Update MountRepos, MigrateConfigAndStatusCacheFiles, WriteOnDiskVersion16CapableFile
  to use Current junction paths
- CurStepChanged: Remove staging logic, call CreateOrUpdateCurrentJunction + GarbageCollectOldVersions
- CurUninstallStepChanged: Remove {app}\Current from PATH instead of {app}

Flat-layout migration stub added (detects {app}\GVFS.exe, logs version) but defers
actual file move to future PR to reduce complexity.

Assisted-by: Claude Sonnet 4.5
Signed-off-by: Tyrie Vella <tyrielv@gmail.com>

Author: tyrielv