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

src/lib/boards.ts

@@ -1,6 +1,17 @@
 import type { BoardDef } from "./types";
 
 export const boards: BoardDef[] = [
+  {
+    id: "elecrow_thinknode_m2",
+    name: "ELECROW ThinkNode-M2",
+    mcu: "ESP32-S3",
+    radio: "SX1262",
+    family: "esp32s3",
+    flashMethod: "download-only",
+    assetPattern: "donglora-elecrow_thinknode_m2-v{version}.bin",
+    notes:
+      "Elecrow Meshtastic handset with SH1106 OLED. Uses CP2102 USB-to-UART bridge — download the .bin and flash with espflash CLI.",
+  },
   {
     id: "heltec_v3",
     name: "Heltec V3",

src/pages/index.astro

@@ -223,7 +223,12 @@ your code ──► client lib ──┘</pre>
   <!-- Supported Boards -->
   <section class="py-12">
     <h2 class="font-mono text-2xl font-bold mb-6 text-text">Supported Boards</h2>
-    <div class="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-5 gap-4">
+    <div class="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-4">
+      <div class="bg-bg-card border border-border p-5 rounded">
+        <h3 class="font-mono font-medium text-text mb-2">ELECROW ThinkNode-M2</h3>
+        <p class="text-sm text-text-muted">ESP32-S3 &middot; SX1262</p>
+        <p class="text-xs text-warn mt-2 font-mono">CLI Only</p>
+      </div>
       <div class="bg-bg-card border border-border p-5 rounded">
         <h3 class="font-mono font-medium text-text mb-2">Heltec V3</h3>
         <p class="text-sm text-text-muted">ESP32-S3 &middot; SX1262</p>