feat: let admins choose the PowerShell engine for script tasks

[mirbach]

Summary

• Adds an engine dropdown to the PowerShell task form: Windows PowerShell 5.1 (default), PowerShell 7+ (pwsh), and Auto (use pwsh if available, otherwise fall back to 5.1).
• Reuses the existing script_executable column already used by unix_script tasks — no schema migration needed.
• Validated in validatePowerShellScript, persisted in AddTaskToProfile/UpdateProfileTask, and threaded through to wingetcfg.ExecutePowershellScript for both the ad-hoc "run task now" and scheduled profile-application flows.
• Locale strings added for all supported languages.

Depends on

• feat: add executable param to select PowerShell engine wingetcfg#5 — needs to merge and a new version tagged/pinned in go.mod before this builds.
• feat: support running PowerShell tasks under PowerShell 7+ (pwsh) openuem-agent#208
• feat: pass script_executable through to PowerShell task config openuem-worker#91

Test plan

Verified end-to-end on a real Hyper-V lab (see openuem-agent#208 for full setup): logged into this console build, admitted a real Windows agent, created PowerShell tasks selecting each engine option from the new dropdown, and confirmed the agent executed each one under the correct PowerShell version — including the auto-detect and explicit-failure-when-missing cases.

• go build ./... and templ generate pass
• Dropdown renders with 5.1 selected by default for new tasks, correctly reflects ScriptExecutable when editing existing tasks
• Task creation/update persists the selected engine
• Existing tasks (ScriptExecutable == "") continue to run unchanged under PowerShell 5.1

[mirbach]

powershell (default/5.1) → runs under Windows PowerShell 5.1 ✅
auto, no PS7 installed → falls back to 5.1 ✅
auto, PS7 installed via MSIX/winget → correctly detects and uses pwsh 7.6.4 ✅
pwsh explicit, not installed → clean failure with a clear error message ✅