feat: support multi-format — images, CSV, JSON, TXT + fix PDF embed

- PDF: <iframe> → <embed> (plus compatible webview Tauri)
- Images: .png .jpg .gif .svg .webp .bmp .ico → rendu standalone avec toolbar
- CSV: rendu en tableau HTML avec headers sticky
- JSON: affichage formaté avec indentation (html.escape)
- Texte: .txt .log .yml .yaml .py .js .sh .toml .ini → <pre> mono
- CSS: .image-viewer-body, .csv-table, .text-viewer, .json-viewer, .file-toolbar

frontend/style.css

@@ -8539,6 +8539,97 @@ body.popup-mode .content-area {
   margin-left: 4px;
 }
 
+/* ── Image Viewer ── */
+.image-viewer-body {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  min-height: 200px;
+  background: var(--surface1);
+  padding: 20px;
+}
+.image-viewer-body img {
+  border-radius: 4px;
+  box-shadow: 0 2px 12px rgba(0,0,0,0.15);
+}
+
+/* ── Text / CSV Viewer ── */
+.text-viewer-container {
+  display: flex;
+  flex-direction: column;
+  height: 100%;
+}
+.file-toolbar {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 8px 16px;
+  background: var(--surface1);
+  border-bottom: 1px solid var(--border);
+  font-size: 0.85rem;
+  flex-shrink: 0;
+}
+.file-info {
+  flex: 1;
+  color: var(--text-dim);
+}
+.text-viewer-body {
+  flex: 1;
+  overflow: auto;
+  padding: 16px;
+  background: var(--surface2);
+}
+.text-viewer-body pre {
+  margin: 0;
+  font-family: 'JetBrains Mono', 'Fira Code', 'Consolas', monospace;
+  font-size: 0.85rem;
+  line-height: 1.5;
+  white-space: pre-wrap;
+}
+.text-viewer-body code {
+  color: var(--text);
+}
+
+/* ── CSV Table ── */
+.csv-table-wrapper {
+  overflow: auto;
+  max-height: 70vh;
+}
+.csv-table {
+  width: 100%;
+  border-collapse: collapse;
+  font-size: 0.85rem;
+}
+.csv-table th {
+  background: var(--surface1);
+  color: var(--text-dim);
+  font-weight: 600;
+  padding: 8px 12px;
+  text-align: left;
+  border-bottom: 2px solid var(--border);
+  position: sticky;
+  top: 0;
+}
+.csv-table td {
+  padding: 6px 12px;
+  border-bottom: 1px solid var(--border-light, var(--border));
+}
+.csv-table tr:hover td {
+  background: var(--surface1);
+}
+
+/* ── JSON Viewer ── */
+.json-viewer {
+  font-family: 'JetBrains Mono', 'Fira Code', 'Consolas', monospace;
+  font-size: 0.85rem;
+  line-height: 1.5;
+  padding: 16px;
+  background: var(--surface2);
+  border-radius: 6px;
+  overflow: auto;
+  max-height: 70vh;
+}
+
 /* ── PaneManager — Split View ── */
 .pane-grid {
   display: grid;