feat: Add repair/reinstall capability for master Steam prefix

[weter11]

Summary

This PR adds repair/reinstall capability for the master Windows Steam prefix, addressing the issue where corrupted Steam installs could not be recovered through the UI.

Changes

1. Repair Logic (src/launch/mod.rs)

• Added repair_master_steam() function that:
  • Stops running Steam/wine processes in master prefix
  • Backs up existing prefix to master_steam_prefix.bak.<timestamp>
  • Keeps only 1 previous backup (deletes oldest before creating new)
  • Re-runs install_master_steam() against fresh prefix

2. UI Components (src/ui.rs)

• Added "Repair / Reinstall Windows Steam Runtime" button in Settings
• Shows confirmation dialog warning about Steam login state before proceeding
• Displays progress/status updates via AsyncOp channel

3. Post-Launch Grace Period Health Check (src/infra/runners/wine_tkg.rs)

• Added 2-second grace period after ready-signal detection
• Polls try_wait() to verify Steam process is still alive
• Detects false-positive scenario where ready-signal exists but process has crashed
• Error message suggests using the repair action

4. Unit Test (src/launch/verification_tests.rs)

• Added MockSteamFalsePositiveRunner and test for grace-check behavior

Acceptance Criteria Met

• ✅ Repair action exists and is reachable from Settings panel with distinct label
• ✅ Confirmation dialog warns about Steam login state before proceeding
• ✅ Post-ready grace-check distinguishes healthy process from exited-process scenario
• ✅ Error message suggests using the repair action
• ✅ Existing install button behavior unchanged