fix(app): harden the setup key imports

- PGP key import reads at most 1 MiB (bounded read, buffer zeroed) and
  refuses larger files with a new message.
- SshKeyViewModel catches any Exception from import and generation
  (CancellationException rethrown) instead of a short list, so an
  unexpected provider or sshj error no longer crashes the app.
- Pending PGP key bytes are zeroed in a finally block, also on
  cancellation during addKey after a replace, and an earlier pending
  pick is zeroed before a new one takes its place.
- PGPKeyManager never replaces a stored secret key with a public-only
  key of the same ID, with or without replace (KeyDowngradeException);
  the setup ViewModel shows a localized message for it. Unit test added.
- storeRecipients strips '#' comments and a trailing '!' as pass reads
  .gpg-id; tests added.
- GitServerViewModel.clearStore returns the failure when the store
  folder cannot be emptied, so the clone does not start; the plan's
  Task 3 call site reports it.
- readGpgId opens the file with NOFOLLOW_LINKS and reads at most
  64 KiB + 1 bytes, refusing more.
- SshKey.import writes the new private key to a temp file and moves it
  over atomically before dropping the old key's Keystore entry, public
  key and passphrase, so an I/O error keeps the old key.
- KeyUtils.isSecretKey notes (ponytail) that GnuPG stub and smartcard
  keyrings count as secret.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index d53700a8..8c046ca5 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -407,4 +407,7 @@
   <string name="ssh_key_state_imported">Imported private key</string>
   <string name="ssh_key_state_generated">Generated on this device</string>
   <string name="git_clone_progress">Cloning…</string>
+  <string name="git_clone_delete_failed">Could not empty the store folder, so the clone did not start.</string>
+  <string name="pgp_key_import_too_large">The file is larger than 1 MiB: not a PGP key.</string>
+  <string name="pgp_key_import_downgrade">A private key with this ID is already imported. Its public key alone cannot replace it: delete the private key first if you really want to.</string>
 </resources>