mnemonic: more thorough recovery phrase word verification

Rather than displaying a number of random words to be confirmed, all
words are displayed in groups of three, with one of them selected for
user verification.  This means that every word will be displayed, and
one in three will be selected/confirmed by the user.

main/process/mnemonic.c

@@ -298,31 +298,26 @@ static bool display_confirm_mnemonic(const size_t nwords, char* mnemonic, const
             }
         }
 
-        // Large enough for 12 and 24 word mnemonic
-        bool already_confirmed[MNEMONIC_MAXWORDS] = { false };
-
-        // Confirm the mnemonic - the number of words to confirm
-        // and the number of options presented for each word.
-        const size_t num_words_confirm = nwords == MNEMONIC_MAXWORDS ? 6 : 4;
+        // Confirm the mnemonic - show groups of three consecutive words
+        // and have user confirm one of them at random.
+        // Ensures all words are at the very least displayed.
+        mnemonic_confirmed = true; // will be set to false if wrong word selected
         const size_t num_words_options = nwords == MNEMONIC_MAXWORDS ? 8 : 6;
-        for (size_t i = 0; i < num_words_confirm; ++i) {
-            size_t selected;
-            do {
-                selected = 1 + get_uniform_random_byte(nwords - 2); // never select the first or last word
-            } while (already_confirmed[selected]);
-            already_confirmed[selected] = true;
-
-            // Make the word to confirm the 'middle' word of the three shown
+        for (size_t i = 0; i < nwords; i += 3) {
+            const size_t offset_word_to_confirm = get_uniform_random_byte(3);
+            const size_t selected = i + offset_word_to_confirm;
             gui_view_node_t* textbox = NULL;
             gui_activity_t* const confirm_act
-                = make_confirm_mnemonic_word_activity(&textbox, selected - 1, 1, words, nwords);
+                = make_confirm_mnemonic_word_activity(&textbox, i, offset_word_to_confirm, words, nwords);
             JADE_LOGD("selected = %u", selected);
 
+            // Pick some other words from the mnemonic as options, but avoid
+            // the words currently displayed on screen (neighbouring words).
             // Large enough for 12 and 24 word mnemonic
             bool already_picked[MNEMONIC_MAXWORDS] = { false };
-            already_picked[selected] = true;
-            already_picked[selected - 1] = true;
-            already_picked[selected + 1] = true;
+            already_picked[i] = true;
+            already_picked[i + 1] = true;
+            already_picked[i + 2] = true;
 
             // Large enough for 12 and 24 word mnemonic
             // (Only really needs to be as big as 'num_words_options' so MAXWORDS is plenty)
@@ -375,9 +370,6 @@ static bool display_confirm_mnemonic(const size_t nwords, char* mnemonic, const
                 await_error_activity("\n Incorrect. Check your\n  recovery phrase and\n           try again.");
                 mnemonic_confirmed = false;
                 break;
-            } else if (i == num_words_confirm - 1) { // last word, and it's correct
-                mnemonic_confirmed = true;
-                break;
             }
         }
     }