chore(sdk): drop private system-back routing patch, restoring LightActivity to upstream

[tyleryancey]

What

Removes the local patch inside LightActivity's OnBackPressedCallback. handleOnBackPressed() is now exactly upstream's version:

override fun handleOnBackPressed() {
    goBack()
}

sdk/client/src/main/kotlin/com/thelightphone/sdk/LightActivity.kt is now byte-identical to upstream/main — git diff upstream/main -- <that path> is empty.

Why

Part of dropping private SDK patches so this repo syncs with upstream without conflicts.

• Accepted behavior loss, documented in light-workspace's upstream backlog rather than filed upstream now. Multi-step screens (e.g. AddEntry's category grid -> amount entry) are popped outright on system back, dropping in-progress input; the on-screen back button still behaves correctly.
• The patch could never ship anyway: Light's builder extracts only tool/** and compiles against Light's official SDK, so nothing under sdk/ reaches a published build.
• light-sudoku's variant of the same fix was already removed.

Scope

One file, one hunk. Deliberately untouched load-bearing deltas: LightDb.kt's destructiveMigration param (three tool call sites depend on it), the LightSdkPlugin isUnitTestConfig exemption, and the lint-rules lint-api pin.

Compilation is unaffected — the ViewModels override onBackPressed(), an upstream hook that still exists; only this call site goes away.

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