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/contextMenus.js

@@ -88,9 +88,8 @@ const contextMenus = {
                 })
                     .then(response => response.json())
                     .then(fileDetails => {
-                        // Check if viewable file type
-                        if ((fileDetails.mime_type && fileDetails.mime_type.startsWith('image/')) || 
-                            (fileDetails.mime_type && fileDetails.mime_type === 'application/pdf')) {
+                        // Check if viewable file type (images, PDFs, text files)
+                        if (window.ui && window.ui.isViewableFile(fileDetails)) {
                             // Open with inline viewer
                             if (window.inlineViewer) {
                                 window.inlineViewer.openFile(fileDetails);