v3.3.0: Welcome screen, factory reset, calibration/touch fixes, flash partition fix

## New
- "Welcome" screen shown on first-time setup (before touch
  calibration), with the splash screen's animated radar graphic and a
  Start button.
- The "Setup complete" screen now counts down 7 seconds with a button
  matching the Welcome screen's style, then automatically continues
  into the app (previously required a tap).
- System menu "Backup & Reset" page (Backup, Restore, and a new
  "Reset settings" option that wipes the on-device data and re-runs
  first-time setup - useful for testing or handing the device to
  someone else).
- Touch calibration targets are now green mini radar reticles instead
  of red crosshairs.

## Improved
- The SD card's data folder is now only created after tapping Start
  on the Welcome screen, instead of on every boot regardless.
- Flash partition layout changed to free up roughly 850 KB of
  firmware space (no OTA update slot needed, since firmware is always
  flashed via cable/USB) - lowers flash usage from ~95% to ~59%.

## Fixed
- The Start button and the WiFi scanning screen now show a loading
  indicator instead of appearing frozen during SD-card/WiFi
  operations.
- A touch-input bug during first-time setup that could corrupt touch
  calibration and cause taps to land on the wrong on-screen element
  afterward (most noticeable in the WiFi network selection list).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/main.cpp

@@ -456,10 +456,21 @@ void setup() {
         haltWithSdRequiredScreen();
         return;
     }
-    SdStorage::seedDefaultDataFiles();
 
     bool isFirstRun = !SD.exists(Config::SD_SETTINGS_FILE);
 
+    // The Flightradar folder structure and default data files are
+    // deliberately NOT created yet on a first boot - that happens further
+    // down, after the Start button on the Welcome screen has been tapped
+    // (the button is "the gate to the app": nothing should exist on the
+    // card before that). On every later boot the structure already
+    // exists anyway - creating it immediately here is harmless,
+    // ensureDir()/writeIfAbsent() are idempotent.
+    if (!isFirstRun) {
+        SdStorage::createStructure();
+        SdStorage::seedDefaultDataFiles();
+    }
+
     SettingsStore::load();
     tft.invertDisplay(SettingsStore::displayInverted());
     // Der erste ledcWrite() oben (vor SettingsStore::load()) kannte die
@@ -488,6 +499,12 @@ void setup() {
     // da die Sprachauswahl (FirstRunLanguageScreen) erst danach kommt,
     // siehe first_run_welcome_screen.cpp.
     if (isFirstRun) {
+        // Creating the folder structure, seeding default data files, AND
+        // showing the loading indicator during those (noticeably slow) SD
+        // accesses now all happen directly inside
+        // FirstRunWelcomeScreen::run() (triggered by the Start tap) - see
+        // there for why (button otherwise looked frozen, repeated tapping
+        // let a tap leak into calibration).
         FirstRunWelcomeScreen::run(tft);
     }