feat(model-manager): add CivitAI browse view + fix civitai.red / 401

- New "Browse CivitAI" view: thumbnail grid with search, type/sort/period
  filters and NSFW toggle; click a model card to download it (per-card
  version picker for multi-version models). Cursor + page based "Load more".
- Backend: /api/civitai/search and /api/civitai/download endpoints; new
  civitai_search() catalog helper.
- Fix 401 on paste: recognize the civitai.red mirror (and any civitai.*
  host), normalize API calls to civitai.com, and always resolve the
  model-version so type + filename are auto-detected for every CivitAI URL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

model-manager/app/static/index.html

@@ -20,6 +20,9 @@
         <button class="nav-item active" data-view="installed">
           <span class="nav-ico">▦</span> Installed Models
         </button>
+        <button class="nav-item" data-view="browse">
+          <span class="nav-ico">🔍</span> Browse CivitAI
+        </button>
         <button class="nav-item" data-view="download">
           <span class="nav-ico">↓</span> Add / Download
         </button>
@@ -45,6 +48,53 @@
         </div>
       </section>
 
+      <!-- Browse CivitAI -->
+      <section class="view" id="view-browse">
+        <header class="view-head">
+          <h1>Browse CivitAI</h1>
+          <div class="head-actions">
+            <span class="form-msg" id="browseToast"></span>
+          </div>
+        </header>
+
+        <div class="browse-controls">
+          <input type="search" id="bSearch" placeholder="Search models…" />
+          <select id="bType">
+            <option value="">All types</option>
+            <option value="Checkpoint">Checkpoint</option>
+            <option value="LORA">LoRA</option>
+            <option value="LoCon">LyCORIS</option>
+            <option value="TextualInversion">Embedding</option>
+            <option value="Controlnet">ControlNet</option>
+            <option value="VAE">VAE</option>
+            <option value="Upscaler">Upscaler</option>
+            <option value="Hypernetwork">Hypernetwork</option>
+            <option value="Poses">Poses</option>
+          </select>
+          <select id="bSort">
+            <option>Most Downloaded</option>
+            <option>Highest Rated</option>
+            <option>Newest</option>
+          </select>
+          <select id="bPeriod">
+            <option>AllTime</option>
+            <option>Year</option>
+            <option>Month</option>
+            <option>Week</option>
+            <option>Day</option>
+          </select>
+          <label class="check"><input type="checkbox" id="bNsfw" /> NSFW</label>
+          <button class="btn primary" id="bGo">Search</button>
+        </div>
+
+        <div id="browseGrid" class="browse-grid">
+          <p class="empty">Search the CivitAI catalog, then click <b>Download</b> on a model.</p>
+        </div>
+        <div class="browse-more">
+          <button class="btn" id="bMore" style="display:none">Load more</button>
+        </div>
+      </section>
+
       <!-- Add / Download -->
       <section class="view" id="view-download">
         <header class="view-head"><h1>Add / Download Model</h1></header>