Use local time for local-backup snapshot folder names

[patjackson52]

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• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• Pixel 10 Pro, Android 17 (playStagingSpinner) — on-device backup run against this branch and against main
• Robolectric: ArchiveFileSystemTest, 14/14

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Description

Fixes #14866.

Snapshot directories for on-device backups are stamped in UTC, so a backup taken at 11:55 local shows up as signal-backup-2026-08-03-18-55-33. The reporter noticed this is a change from the v1 .backup files, which used local time.

This appears to be the only place in the app that stamps a filename in UTC. Android's v1 backups both write and parse in device-local time (LocalBackupJob / LocalBackupJobApi29, and BackupUtil.getBackupTimestamp, none of which set an explicit zone), and iOS's local backups do the same — LocalFileBackupManager.Constants.dateFormatter sets no timeZone, so it uses TimeZone.current. That's what led me to read v2's UTC as unintentional. If it was deliberate, I'm happy to close this and instead look at displaying local time wherever these names surface to users — just say the word.

Two changes:

• createSnapshot() names new snapshots in the device's local time zone.
• listSnapshots() no longer derives ordering from names. Directories written before this change carry UTC names, and nothing in a name distinguishes the two generations, so parsing a mixed directory with one rule orders an older snapshot ahead of a newer one by the local UTC offset. That ordering drives deleteOldBackups() retention, and RestoreLocalBackupViewModel renders SnapshotInfo.timestamp as the date label when picking a backup to restore, so a legacy directory written at 8pm local would also display under the following day's date. It now uses the directory's last-modified time, falling back to the parsed name only when a provider reports no modified time.

Renaming in either direction has the mixed-generation problem, which is why ordering moves off names rather than trying to parse both formats.

Testing

Built this branch and main onto a Pixel 10 Pro and ran an on-device backup with each, ~9 minutes apart in America/Los_Angeles:

signal-backup-2026-08-03-12-04-35   <- this branch, phone clock read 12:04 PDT
signal-backup-2026-08-03-18-55-33   <- main,        phone clock read 11:55 PDT

That pair is also what the new ordering test reconstructs: a mixed UTC/local directory asserting the newer snapshot sorts first, which fails without the listSnapshots() change. Also added a round-trip test pinned to a non-UTC zone (fails if either the write or parse side alone reverts to UTC) and a test for the name-parsing fallback. ArchiveFileSystemTest passes 14/14.

Unrelated observation

iOS writes signal-backups-<date> (plural) while Android writes signal-backup-<date> (singular), and the iOS doc comment documents the singular form. Android's prefix filter matches both by accident. Not touching it here, but flagging it in case local backup directories are meant to be portable between platforms.