feat: add Waveshare RP2040-LoRa board support

New RP2040 board family in board selector, homepage grid, and UF2
download flow (RPI-RP2 drive name, correct probe-rs chip).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/BoardSelector.svelte

@@ -10,6 +10,7 @@
   let { selected, onselect }: Props = $props();
 
   const espBoards = boards.filter((b) => b.family === "esp32s3");
+  const rpBoards = boards.filter((b) => b.family === "rp2040");
   const nrfBoards = boards.filter((b) => b.family === "nrf52840");
 
   function badgeText(board: BoardDef): string {
@@ -55,6 +56,23 @@
     {/each}
   </div>
 
+  <p class="text-xs text-text-dim mb-2 font-mono">RP2040</p>
+  <div class="grid grid-cols-1 sm:grid-cols-2 gap-3 mb-4">
+    {#each rpBoards as board}
+      <button
+        class="text-left bg-bg-card border p-4 rounded transition-all cursor-pointer
+          {selected?.id === board.id
+          ? 'border-accent shadow-[0_0_12px_oklch(from_var(--color-accent)_l_c_h_/_0.15)]'
+          : 'border-border hover:border-border-bright'}"
+        onclick={() => onselect(board)}
+      >
+        <div class="font-mono font-medium text-text text-sm">{board.name}</div>
+        <div class="text-xs text-text-muted mt-1">{board.mcu} &middot; {board.radio}</div>
+        <div class="text-xs {badgeColor(board)} mt-2 font-mono">{badgeText(board)}</div>
+      </button>
+    {/each}
+  </div>
+
   <p class="text-xs text-text-dim mb-2 font-mono">nRF52840</p>
   <div class="grid grid-cols-1 sm:grid-cols-2 gap-3">
     {#each nrfBoards as board}