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>

frontend/index.html

@@ -110,14 +110,32 @@
       <div class="alert alert-info" style="margin-bottom:16px;">
         <span>ℹ</span>
         <div>
-          Arduino IDE <strong>Sketch → Export Compiled Binary</strong> 로 생성한 .bin 파일을 업로드하세요.
-          <br>병합 바이너리(merged.bin)는 '펌웨어' 하나만 업로드해도 됩니다.
+          Arduino IDE <strong>Sketch → Export Compiled Binary</strong> 후 빌드 폴더에서 파일을 선택하세요.
           <br><small style="color:var(--text-muted);">
-            분리 파일 사용 시: bootloader(0x0000) + partition-table(0x8000) + app(0x10000)
+            병합 바이너리: <code>*.merged.bin</code> 하나만 업로드 (권장) |
+            분리 파일: bootloader + partitions + boot_app0 + app
           </small>
         </div>
       </div>
 
+      <!-- 업로드 방식 선택 -->
+      <div style="display:flex;gap:10px;margin-bottom:18px;">
+        <label id="lbl-merged" style="flex:1;display:flex;align-items:center;gap:10px;padding:12px 16px;border:2px solid var(--accent);border-radius:8px;cursor:pointer;background:rgba(0,200,150,.06);">
+          <input type="radio" name="upload-mode" id="mode-merged" value="merged" checked style="accent-color:var(--accent);">
+          <div>
+            <div style="font-size:13px;font-weight:600;">병합 바이너리 <span style="color:var(--accent);font-size:11px;">권장</span></div>
+            <div style="font-size:11px;color:var(--text-muted);">*.merged.bin 하나 업로드 — offset 0x0</div>
+          </div>
+        </label>
+        <label id="lbl-split" style="flex:1;display:flex;align-items:center;gap:10px;padding:12px 16px;border:2px solid var(--border);border-radius:8px;cursor:pointer;">
+          <input type="radio" name="upload-mode" id="mode-split" value="split" style="accent-color:var(--accent);">
+          <div>
+            <div style="font-size:13px;font-weight:600;">분리 파일</div>
+            <div style="font-size:11px;color:var(--text-muted);">bootloader + partitions + boot_app0 + app</div>
+          </div>
+        </label>
+      </div>
+
       <form id="upload-form">
         <div class="form-grid">
           <div class="form-group">
@@ -161,40 +179,49 @@
         </div>
 
         <!-- 드롭존: 펌웨어 (필수) -->
-        <div class="form-group">
-          <label>펌웨어 바이너리 <span style="color:var(--danger);">*</span></label>
+        <div class="form-group" id="zone-firmware">
+          <label id="lbl-firmware">펌웨어 바이너리 <span style="color:var(--danger);">*</span></label>
           <div class="drop-zone" id="drop-firmware">
             <input type="file" id="file-firmware" accept=".bin" />
             <div class="icon">📦</div>
-            <div class="drop-label">
-              펌웨어 .bin 파일을 드래그하거나 클릭하세요
-              <div style="font-size:11px;margin-top:4px;color:var(--text-muted);">(최대 8 MB)</div>
+            <div class="drop-label" id="firmware-hint">
+              <span id="firmware-hint-text">*.merged.bin 파일을 드래그하거나 클릭하세요</span>
+              <div style="font-size:11px;margin-top:4px;color:var(--text-muted);">(최대 32 MB)</div>
             </div>
           </div>
         </div>
 
-        <!-- 분리 파일 섹션 (선택) -->
-        <details style="margin-bottom:14px;">
-          <summary style="cursor:pointer;font-size:13px;color:var(--text-muted);margin-bottom:8px;">
-            ▸ 분리 바이너리 파일 추가 (선택)
-          </summary>
-          <div style="padding-top:10px;display:flex;flex-direction:column;gap:12px;">
-            <div class="form-group" style="margin:0;">
-              <label>부트로더 (bootloader.bin) — offset 0x0000</label>
-              <div class="drop-zone" id="drop-bootloader" style="padding:16px;">
-                <input type="file" id="file-bootloader" accept=".bin" />
-                <div class="drop-label">부트로더 .bin (선택사항)</div>
-              </div>
-            </div>
-            <div class="form-group" style="margin:0;">
-              <label>파티션 테이블 (partition-table.bin) — offset 0x8000</label>
-              <div class="drop-zone" id="drop-partitions" style="padding:16px;">
-                <input type="file" id="file-partitions" accept=".bin" />
-                <div class="drop-label">파티션 테이블 .bin (선택사항)</div>
-              </div>
+        <!-- 분리 파일 섹션 (mode-split 선택 시 표시) -->
+        <div id="split-files" style="display:none;flex-direction:column;gap:12px;margin-bottom:14px;">
+          <div class="form-group" style="margin:0;">
+            <label>① 부트로더 — offset 0x0000 <span style="color:var(--danger);">*</span></label>
+            <div class="drop-zone" id="drop-bootloader" style="padding:16px;">
+              <input type="file" id="file-bootloader" accept=".bin" />
+              <div class="drop-label">*.bootloader.bin 드래그 또는 클릭</div>
             </div>
           </div>
-        </details>
+          <div class="form-group" style="margin:0;">
+            <label>② 파티션 테이블 — offset 0x8000 <span style="color:var(--danger);">*</span></label>
+            <div class="drop-zone" id="drop-partitions" style="padding:16px;">
+              <input type="file" id="file-partitions" accept=".bin" />
+              <div class="drop-label">*.partitions.bin 드래그 또는 클릭</div>
+            </div>
+          </div>
+          <div class="form-group" style="margin:0;">
+            <label>③ Boot App0 — offset 0xe000 <span style="color:var(--text-muted);font-size:11px;">(OTA 사용 시 필요)</span></label>
+            <div class="drop-zone" id="drop-boot-app0" style="padding:16px;">
+              <input type="file" id="file-boot-app0" accept=".bin" />
+              <div class="drop-label">boot_app0.bin 드래그 또는 클릭</div>
+            </div>
+          </div>
+          <div class="form-group" style="margin:0;">
+            <label>④ 애플리케이션 — offset 0x10000 <span style="color:var(--danger);">*</span></label>
+            <div class="drop-zone" id="drop-app" style="padding:16px;">
+              <input type="file" id="file-app" accept=".bin" />
+              <div class="drop-label">*.ino.bin 드래그 또는 클릭</div>
+            </div>
+          </div>
+        </div>
 
         <div id="upload-progress" style="display:none;margin-bottom:14px;">
           <div class="progress-wrap"><div class="progress-bar" id="upload-bar"></div></div>