feat: add ELECROW ThinkNode-M2 board support

ESP32-S3 Meshtastic handset with SH1106 OLED. Flashes via the
CP2102 bridge, so this board is download-only like heltec_v3_uart —
Web Serial is unreliable through the bridge. The existing
`download-only` branch of EspFlashFlow surfaces the CP2102 warning
plus a .bin download and espflash CLI instructions, so no new
component code is needed.

ESP32-S3 grid in BoardSelector goes from sm:grid-cols-3 to
sm:grid-cols-2 lg:grid-cols-4 to accommodate the fourth ESP board.
Homepage Supported Boards grid drops from lg:grid-cols-5 to
lg:grid-cols-3 (3x2 layout) — six 200-px cards in one row was tight.

src/components/BoardSelector.svelte

@@ -40,7 +40,7 @@
   <h3 class="font-mono text-sm text-text-muted mb-3 uppercase tracking-wider">Select Board</h3>
 
   <p class="text-xs text-text-dim mb-2 font-mono">ESP32-S3</p>
-  <div class="grid grid-cols-1 sm:grid-cols-3 gap-3 mb-4">
+  <div class="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-4 gap-3 mb-4">
     {#each espBoards as board}
       <button
         class="text-left bg-bg-card border p-4 rounded transition-all cursor-pointer