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/sd_storage.cpp

@@ -108,16 +108,19 @@ bool init() {
     sdSpi.begin(Config::SD_SPI_CLK_PIN, Config::SD_SPI_MISO_PIN,
                 Config::SD_SPI_MOSI_PIN, Config::SD_SPI_CS_PIN);
     mounted = SD.begin(Config::SD_SPI_CS_PIN, sdSpi, 4000000);
-    if (!mounted) return false;
-
-    ensureDir(Config::SD_ROOT_DIR);
-    ensureDir(Config::SD_LOG_DIR);
-    ensureDir(Config::SD_SCREENSHOT_DIR);
-    return true;
+    return mounted;
 }
 
 bool isMounted() { return mounted; }
 
+void createStructure() {
+    if (!mounted) return;
+    SdMutex::Guard guard;
+    ensureDir(Config::SD_ROOT_DIR);
+    ensureDir(Config::SD_LOG_DIR);
+    ensureDir(Config::SD_SCREENSHOT_DIR);
+}
+
 void seedDefaultDataFiles() {
     if (!mounted) return;
     SdMutex::Guard guard;