Fix clearVault to return actual deletion outcome instead of always true

[Copilot]

VaultStorage.clearVault() unconditionally returned true, making it impossible for callers to distinguish between "vault existed and was deleted" vs "vault never existed" — contradicting VaultManager.clearVault()'s KDoc which documents false for the latter case.

Change

• VaultStorage.clearVault() now checks vault existence before purging and returns the real outcome:

fun clearVault(context: Context, appId: String): Boolean {
    val existed = vaultExists(context, appId)
    purgeVault(context, appId)
    return existed
}

true → vault existed and was removed; false → vault was not present. Behavior is now consistent with documented semantics in VaultManager.

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