fix(android): keep the ETag only when the decryption that followed succeeded

The harness stored the ETag on download, so a first attempt with no password left a valid tag behind: the next call answered 304 and there was nothing to decrypt, precisely when the right password had just been typed. Sending If-None-Match without holding the matching plaintext trades a download for having no data - the real client needs the same rule, the tag belonging to the cached vault.

Also drops an em dash from an exception message: Delphi reads a BOM-less source as ANSI and it reached the device as mojibake.

mainApk.pas

@@ -46,7 +46,11 @@ type
     FBtn: TButton;
     FBtnFetch: TButton;
     FUrl, FToken, FPwd: TEdit;
-    FETag: string;   // conserve entre deux appuis : c'est ce qui permet le 304
+    // L'ETag n'est conserve QUE si le dechiffrement qui a suivi a reussi :
+    // envoyer If-None-Match sans detenir le clair correspondant fait
+    // economiser un telechargement pour se retrouver sans donnees. Le vrai
+    // client aura la meme regle, l'ETag etant lie au coffre en cache.
+    FETag: string;
     procedure RunSelfTest(Sender: TObject);
     procedure RunFetch(Sender: TObject);
     procedure Log(const AMsg: string);
@@ -190,17 +194,16 @@ begin
         end;
 
         Log('  recu ' + Length(LRes.Container).ToString + ' octets');
-        if LRes.ETag <> '' then
-        begin
-          FETag := LRes.ETag;
-          Log('  ETag conserve : ' + FETag);
-        end;
 
         Log('');
         Log('Dechiffrement...');
         LSw := TStopwatch.StartNew;
+        // Volontairement AVANT de retenir l'ETag : un echec ici doit laisser
+        // FETag vide pour que le prochain essai retelecharge.
         LPlain := DecryptContainer(LRes.Container, LPwd);
         LSw.Stop;
+        FETag := LRes.ETag;
+        if FETag <> '' then Log('  ETag retenu : ' + FETag);
         Log('  ' + LSw.ElapsedMilliseconds.ToString + ' ms pour ' +
             Length(LPlain).ToString + ' caracteres');