[ProjectZero02]

Right after 2.2.1 was published, the phone's in-app update failed with "The download is corrupt (checksum mismatch)".

The release is fine. Downloading v2.2.1/PowerDeck.apk here gives the same SHA-256 that GitHub's asset digest and SHA256SUMS.txt list (ebb9c1c5…871a6c), including with OkHttp's request headers through the redirect.

Cause (app side).

• Every tap on Update launched a new coroutine downloading into the same cacheDir/update.apk. That includes the home banner, the Settings row and the Settings button.
• Nothing showed that a download was running, so a second tap was easy.
• Two downloads at once truncated and overwrote each other's file, and the checksum check then failed. A finished one could also delete the file while the other was still reading it.

Fix.

• MainViewModel.installUpdate runs one download at a time with the new state AppCheck.Downloading. The home banner reads "Downloading PowerDeck X…" with no button; Settings shows the same with a spinner.
• Updater.download puts each download in a temp file of its own and hashes it as it streams, with no second read of the whole file.
• A download that doesn't match is retried once, and a damaged copy is always deleted.
• The message after two damaged downloads says to tap Update again.

Tests (UpdaterTest, MockWebServer with a throttled 512 KB body):

• a clean download comes back intact, with no stray files;
• a damaged download is retried, and the damaged copy is removed;
• a download damaged twice fails with the checksum message and leaves nothing behind;
• two downloads at once both come back intact in separate files. With the old shared update.apk this test fails, which I checked by putting the old line back.

Android unit tests: 67, all passing, and the screenshots are unchanged.

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