feat: batch URL download for LoRA models

Add multi-URL batch download support to the download modal.
Users can paste multiple CivitAI URLs (one per line) in a textarea,
preview all parsed models in a compact list, optionally change versions
per model, select a unified download path, and batch download sequentially.

Single URL behavior is preserved unchanged.

Changes:
- Replace single-line input with textarea for multi-URL input
- Add batch preview step with compact list (thumbnail, version, size)
- Per-item version editing via existing version selector
- Batch download with WebSocket progress tracking (reuses existing infra)
- URL deduplication by model ID, preserving paste order
- Invalid URLs shown inline with remove option
- Fix: prevent click listener accumulation in showVersionStep

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

templates/components/modals/download_modal.html

@@ -10,7 +10,7 @@
         <div class="download-step" id="urlStep">
             <div class="input-group">
                 <label for="modelUrl" id="modelUrlLabel">{{ t('modals.download.url') }}:</label>
-                <input type="text" id="modelUrl" placeholder="{{ t('modals.download.placeholder') }}" />
+                <textarea id="modelUrl" rows="5" placeholder="{{ t('modals.download.placeholder') }}"></textarea>
                 <div class="error-message" id="urlError"></div>
             </div>
             <div class="modal-actions">
@@ -18,7 +18,18 @@
             </div>
         </div>
 
-        <!-- Step 2: Version Selection -->
+        <!-- Step 2: Batch Preview (multi-URL mode) -->
+        <div class="download-step" id="batchPreviewStep" style="display: none;">
+            <div class="batch-preview-list" id="batchPreviewList">
+                <!-- Batch items will be inserted here dynamically -->
+            </div>
+            <div class="modal-actions">
+                <button class="secondary-btn" id="backToUrlFromBatchBtn">{{ t('common.actions.back') }}</button>
+                <button class="primary-btn" id="nextFromBatchBtn">{{ t('common.actions.next') }}</button>
+            </div>
+        </div>
+
+        <!-- Step 3: Version Selection (single-URL or per-item editing) -->
         <div class="download-step" id="versionStep" style="display: none;">
             <div class="version-list" id="versionList">
                 <!-- Versions will be inserted here dynamically -->