fix: resolve file viewer auth issues and add text file viewing support

- Fix file viewer not sending JWT auth tokens when loading files
  - inlineViewer.js: already used XHR with auth (images/PDFs worked)
  - fileViewer.js: was setting img.src/iframe.src directly without auth headers,
    now uses fetch with Bearer token and blob URLs
  - ui.js/contextMenus.js/fileRenderer.js/recent.js/favorites.js: replaced all
    window.location.href = /api/files/... (unauthenticated navigation) with
    authenticated viewer or fileOps.downloadFile()

- Add text file viewing support (text/*, application/json, etc.)
  - New createTextViewer() in inlineViewer.js with authenticated fetch
  - New loadTextViewer() in fileViewer.js with authenticated fetch
  - New isViewableFile() helper in ui.js used across all entry points
  - CSS styles for .inline-viewer-text-content and .file-viewer-text-content

- Translate remaining Spanish strings to English in viewer files

Fixes: text files showing 'Token not provided', images failing to load,
and text files not being previewable at all.

static/js/fileRenderer.js

@@ -304,8 +304,10 @@ class FileRenderer {
           window.inlineViewer.openFile(item);
         } else {
           console.warn('Inline viewer not available, downloading directly');
-          // Fallback to direct download if viewer is not available
-          window.location.href = `/api/files/${item.id}`;
+          // Fallback to authenticated download if viewer is not available
+          if (window.fileOps) {
+            window.fileOps.downloadFile(item.id, item.name);
+          }
         }
       });
     }
@@ -448,8 +450,10 @@ class FileRenderer {
           window.inlineViewer.openFile(item);
         } else {
           console.warn('Inline viewer not available, downloading directly');
-          // Fallback to direct download if viewer is not available
-          window.location.href = `/api/files/${item.id}`;
+          // Fallback to authenticated download if viewer is not available
+          if (window.fileOps) {
+            window.fileOps.downloadFile(item.id, item.name);
+          }
         }
       });
     }