fix: set sort_order for newly installed apps to avoid random positioning

[rowanchen-com]

What this PR does / why we need it?

Newly installed apps have sort_order defaulting to 0, which causes them to appear at random positions among other apps that also have sort_order = 0. This fix assigns sort_order = max(sort_order) + 1 at install time so new apps always appear at the end of the non-favorite list.

Summary of your change

• In agent/app/service/app.go, query the current max sort_order of non-favorite apps before creating the install record, and set the new app's sort_order to max + 1.

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

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