feat: support merged.bin, boot_app0, upload mode toggle + flash guide

- server.js: add boot_app0 field at 0xe000, raise file limit 8→32 MB
- index.html: add 병합/분리 mode toggle, boot_app0 drop zone, numbered split zones
- app.js: dynamic mode logic, remove hardcoded flashAddress 0x10000,
  server now auto-selects 0x0 (merged) or 0x10000 (split)
- flash-guide.html: step-by-step Korean flash guide with file table,
  method A/B walkthrough, flash_args explanation, troubleshooting

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/server.js

@@ -52,7 +52,7 @@ const upload = multer({
       cb(new Error('.bin 파일만 업로드 가능합니다'));
     }
   },
-  limits: { fileSize: 8 * 1024 * 1024 }, // 8 MB
+  limits: { fileSize: 32 * 1024 * 1024 }, // 32 MB
 });
 
 // ── 라우트 ──────────────────────────────────────────────────────────────────
@@ -68,13 +68,14 @@ app.get('/api/firmware', (_req, res) => {
 });
 
 // 펌웨어 업로드
-// 지원 필드: firmware(필수), bootloader(선택), partitions(선택)
+// 지원 필드: firmware(필수), bootloader(선택), partitions(선택), boot_app0(선택)
 app.post(
   '/api/firmware/upload',
   upload.fields([
-    { name: 'firmware',     maxCount: 1 },
-    { name: 'bootloader',   maxCount: 1 },
-    { name: 'partitions',   maxCount: 1 },
+    { name: 'firmware',   maxCount: 1 },
+    { name: 'bootloader', maxCount: 1 },
+    { name: 'partitions', maxCount: 1 },
+    { name: 'boot_app0',  maxCount: 1 },
   ]),
   (req, res) => {
     try {
@@ -105,7 +106,15 @@ app.post(
         });
       }
 
-      // 부트로더가 따로 없으면 0x0, 있으면 0x10000
+      if (files.boot_app0) {
+        parts.push({
+          file: files.boot_app0[0].filename,
+          offset: '0xe000',
+          label: 'Boot App0',
+        });
+      }
+
+      // 부트로더가 따로 없으면(병합 바이너리) 0x0, 있으면 0x10000
       const appOffset = flashAddress
         || (files.bootloader ? '0x10000' : '0x0000');