Ensure integrity tree is generated before packaging

[Marc-André Moreau (mamoreau-devolutions)]

• I have read the contributing guidelines, and I agree with the Code of Conduct.
• Have you checked that there aren't other open pull requests for the same changes?
• Have you tested that the committed code can be executed without errors?
• This PR is not composed of garbage changes used to farm GitHub activity to enter potential Crypto AirDrops.

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This change ensures IntegrityTree.json is generated and refreshed automatically at the points where packaging can otherwise ship stale or missing hashes.

Changes included:

• add a post-publish integrity tree generation target to the app project
• add a reusable integrity tree validator script
• add a refresh helper that regenerates and validates the staged tree
• refresh the staged tree immediately before packaging in local builds and in the release workflow
• keep signing flow support for refreshing the tree when signing a binary directory

Validation performed:

• local publish plus integrity validation succeeded on the arm64 publish output
• dry-run release workflow succeeded for x64 and arm64 packaging on this branch