feat(android): a local account, so the app serves without a repository

Until now the phone could only read a vault that came from elsewhere:
without a URL, a token and a sync password it showed nothing. It now
signs in like the desktop does - username and master password - and can
create its own vault, with no network at all.

The local container carries exactly what the repository would carry: same
format, same encryption under the password. Two reasons, not one. It is
the desktop's model, where the master password protects the vault at rest
rather than the hardware alone; and the format staying the repository's
means sync keeps working with nothing to convert. No Keystore on that
file - it is already encrypted under the password, and binding it to the
device would make it unrecoverable after a reinstall. vault.bin keeps
that role: it is the fast-open cache, and only it needs sealing.

The password opens the container - the decryption itself is the check, no
secret is ever compared - and the username, read from inside once open,
says whose vault it is.

The home screen now knows which of three situations it is in: nothing
here yet, a container without a cache, or a cache ready to open.

Verified on the device: with the cache set aside, the screen offers
username, master password and Create vault, alongside the download path.
Creating an account could not be exercised to the end - the device locked
its screen mid-run - and the vault that was set aside is back in place,
same size, still sealed under its code.

android-client/PMA.VaultPatch.pas

@@ -35,6 +35,11 @@ type
 /// ecrire quand meme la ferait reapparaitre.
 function PatchEntryField(const APlainJson, AUuid, AField, AValue: string): string;
 
+/// Un instantane vide, au nom de ce compte. Sert a l'inscription depuis le
+/// telephone : meme forme que ce qu'ecrit le poste, pour qu'un coffre ne
+/// naisse pas dans un dialecte a part.
+function NewEmptySnapshot(const AUsername: string): string;
+
 /// Horodatage UTC au format que tout le reste emploie (toISOString cote JS).
 /// MELANGER heure locale et UTC a deja decale l'arbitrage de la fusion une
 /// fois : il n'y a qu'une source d'heure ici, et elle est en UTC.
@@ -48,6 +53,28 @@ begin
     TTimeZone.Local.ToUniversalTime(Now));
 end;
 
+function NewEmptySnapshot(const AUsername: string): string;
+var
+  LRoot: TJSONObject;
+begin
+  LRoot := TJSONObject.Create;
+  try
+    // Les memes cles que buildSyncSnapshot, tableaux vides compris : un lecteur
+    // qui trouve la cle absente et un lecteur qui la trouve vide ne se
+    // comportent pas forcement pareil, et ce coffre sera lu par le poste.
+    LRoot.AddPair('version', TJSONNumber.Create(1));
+    LRoot.AddPair('snapshot_at', NowUtcIso);
+    LRoot.AddPair('username', AUsername);
+    LRoot.AddPair('folders', TJSONArray.Create);
+    LRoot.AddPair('avatar_b64', '');
+    LRoot.AddPair('entries', TJSONArray.Create);
+    LRoot.AddPair('tombstones', TJSONArray.Create);
+    Result := LRoot.ToJSON;
+  finally
+    LRoot.Free;
+  end;
+end;
+
 function PatchEntryField(const APlainJson, AUuid, AField, AValue: string): string;
 var
   LRoot, LEntry: TJSONObject;