Storage: format the SD card as one FAT32 partition

Repartitions the whole card (f_fdisk + f_mkfs) on the storage task.
Temporarily triggered from the diagnostics screen by typing FORMAT and
Enter; moves to Settings > Storage in step 7.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

src/main.cpp

@@ -23,6 +23,7 @@ static PowerService* power;
 static ClockService* clockService;
 
 static String lastKey = "-";
+static String typed;  // last characters typed, for the FORMAT confirmation
 static String lastEvent = "-";
 static bool dirty = true;
 
@@ -55,7 +56,7 @@ static void draw() {
     line("Key: " + lastKey);
     line("Event: " + lastEvent);
     d.setTextColor(TFT_DARKGREY);
-    line("+/- brightness, d: dim test (5s/10s)");
+    line(storage->formatting() ? "Formatting SD card..." : "+/- bright, d: dim test, FORMAT+enter: wipe SD");
 }
 
 static void logEvent(const String& text) {
@@ -103,6 +104,14 @@ static void handleKeys() {
               (state.enter ? " enter" : "") + (state.del ? " del" : "");
     dirty = true;
 
+    if (state.enter) {
+        if (typed.endsWith("FORMAT") && storage->requestFormat()) logEvent("format requested");
+        typed = "";
+    } else {
+        typed += word;
+        if (typed.length() > 16) typed.remove(0, typed.length() - 16);
+    }
+
     int brightness = settings.getInt(Setting::Brightness);
     if (word == "=" || word == "+") settings.setInt(Setting::Brightness, brightness + 10);
     if (word == "-") settings.setInt(Setting::Brightness, brightness - 10);