File manager: selection, keyboard shortcuts, rename modal, security, localization

- Selection model: click=select, dblclick=open, Ctrl+click toggle,
  Shift+click range select, Ctrl+A, Esc to clear
- Keyboard shortcuts: Enter, Ctrl+Enter, F2, Backspace (navigate up),
  Delete/Backspace (delete with confirm)
- Rename modal replaces prompt() with validation via backend ValidateName
- Context menu: Open, Open External, Show in Folder, Rename, Duplicate,
  Copy, Cut, Delete — all with SVG icons and Russian labels
- Backend security: vaultPath/absPathSafe helpers prevent path traversal,
  validateName rejects .. / \ null bytes empty overlong names
- MoveNode auto-renames on name conflict (copy style)
- Duplicate uses (copy) (copy 2) suffix pattern
- Russian localization: all file type labels, preview messages, tooltips
- FilePreviewModal: fixed broken {/if} tag

frontend/src/lib/FilePreviewModal.svelte

@@ -43,7 +43,7 @@
       </div>
       <div class="preview-meta">{formatFileSize(item.size)} · {formatMimeType(item.mime)}</div>
       <div class="preview-actions">
-        <button class="action-btn" on:click={handleOpenExternal} title="Open in external program" aria-label="Open externally">
+        <button class="action-btn" on:click={handleOpenExternal} title="Открыть во внешней программе" aria-label="Открыть внешне">
           <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
             <path d="M18 13v6a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h6"/>
             <polyline points="15 3 21 3 21 9"/>
@@ -60,11 +60,11 @@
     </header>
     <div class="preview-body">
       {#if loading}
-        <div class="preview-status"><p>Loading preview...</p></div>
+        <div class="preview-status"><p>Загрузка...</p></div>
       {:else if error}
         <div class="preview-status">
           <p>{error}</p>
-          <button class="btn btn-sm" on:click={handleOpenExternal}>Open in external program</button>
+          <button class="btn btn-sm" on:click={handleOpenExternal}>Открыть во внешней программе</button>
         </div>
       {:else if showImage && content}
         <div class="preview-image-container">
@@ -79,14 +79,14 @@
           </div>
         {:else}
           <div class="preview-status">
-            <p>PDF preview is not available in this environment.</p>
-            <button class="btn btn-sm" on:click={handleOpenExternal}>Open in external program</button>
+            <p>Предпросмотр PDF недоступен.</p>
+            <button class="btn btn-sm" on:click={handleOpenExternal}>Открыть во внешней программе</button>
           </div>
         {/if}
       {:else}
         <div class="preview-status">
-          <p>Preview is not available for this file type.</p>
-          <button class="btn btn-sm" on:click={handleOpenExternal}>Open in external program</button>
+          <p>Предпросмотр недоступен для этого типа файлов.</p>
+          <button class="btn btn-sm" on:click={handleOpenExternal}>Открыть во внешней программе</button>
         </div>
       {/if}
     </div>