feat: entry previews — tweet/thread/article/video/audio/image/iframe/pdf

- PreviewModal: modal overlay with new-tab link (↗) and keyboard close
- PreviewPanel: routes by entity_kind + primary_media extension to the
  correct viewer (tweet/video/audio/pdf/html/image/fallback)
- TweetPreview: full X-style tweet, thread, and article renderer with
  local artifact map for archived media (CDN fallback)
- AudioBar: persistent fixed bottom player, triggered via ContextRail
  Play button; body.has-audio-bar pads content above it
- VideoPreview, IframePreview, ImagePreview: inline viewers
- PreviewPage: standalone /preview/:archiveId/:entryUid route
- ContextRail: Play/Preview buttons; isAudio/isPreviewable detection
- App.jsx: preview modal state, currentAudio state, preview route guard,
  has-audio-bar body class effect
- routes.rs: CSP updated (media-src self blob https; frame-ancestors self;
  Google Fonts + external images/scripts whitelisted)
- styles.css: preview modal, tweet-wrap scroll (min-height:0), audio bar
  body padding, newtab button, preview panel flex layout

frontend/src/styles.css

@@ -1906,29 +1906,11 @@ select {
    ENTRY PREVIEWS
    ════════════════════════════════════════════════════════════════════ */
 
-/* ── 3-column layout when preview is active ─────────────────────── */
-.app-shell.has-preview {
-  grid-template-columns: minmax(0, 1fr) 300px 340px;
-}
-.app-shell.audio-playing {
-  padding-bottom: 72px;
-}
-
-/* ── Preview pane ─────────────────────────────────────────────────── */
-.preview-pane {
-  min-width: 0;
-  overflow: auto;
-  border-left: 1px solid var(--line);
-  background: var(--paper);
-  display: flex;
-  flex-direction: column;
-}
 .preview-panel {
   flex: 1;
   display: flex;
   flex-direction: column;
   min-height: 0;
-  height: 100%;
 }
 .preview-panel-loading,
 .preview-panel-empty {
@@ -1966,7 +1948,6 @@ select {
   display: flex;
   flex-direction: column;
   min-height: 0;
-  height: 100%;
 }
 .preview-iframe-toolbar {
   display: flex;
@@ -2029,8 +2010,9 @@ select {
 /* ── Tweet preview wrap ───────────────────────────────────────────── */
 .preview-tweet-wrap {
   flex: 1;
+  min-height: 0;
   overflow-y: auto;
-  padding: 20px 16px;
+  padding: 12px 16px;
   background: var(--paper-2);
 }
 
@@ -2340,15 +2322,102 @@ select {
 }
 .audio-bar-close:hover { background: var(--field); color: var(--ink); }
 
-/* ── Responsive: hide preview on small screens ─────────────────── */
-@media (max-width: 1100px) {
-  .app-shell.has-preview {
-    grid-template-columns: minmax(0, 1fr) 340px;
-  }
-  .preview-pane { display: none; }
+
+/* ── Preview modal ────────────────────────────────────────────────── */
+.preview-modal-backdrop {
+  position: fixed;
+  inset: 0;
+  z-index: 200;
+  background: rgba(0, 0, 0, 0.55);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 20px;
 }
-@media (max-width: 900px) {
-  .app-shell.has-preview {
-    grid-template-columns: 1fr;
-  }
+.preview-modal {
+  background: var(--paper);
+  border-radius: 12px;
+  box-shadow: 0 24px 80px rgba(0, 0, 0, 0.28);
+  width: 90vw;
+  max-width: 1100px;
+  height: 88vh;
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
 }
+.preview-modal-header {
+  display: flex;
+  align-items: center;
+  padding: 14px 18px;
+  border-bottom: 1px solid var(--line);
+  flex-shrink: 0;
+  gap: 12px;
+}
+.preview-modal-title {
+  flex: 1;
+  min-width: 0;
+  font-size: 14px;
+  font-weight: 600;
+  color: var(--ink);
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+.preview-modal-close {
+  width: 30px;
+  height: 30px;
+  border-radius: 50%;
+  border: none;
+  background: transparent;
+  color: var(--muted-2);
+  font-size: 20px;
+  line-height: 1;
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  flex-shrink: 0;
+  transition: background 0.12s, color 0.12s;
+}
+.preview-modal-close:hover { background: var(--field); color: var(--ink); }
+.preview-modal-body {
+  flex: 1;
+  min-height: 0;
+  overflow: auto;
+  display: flex;
+  flex-direction: column;
+}
+/* Preview button in context rail */
+.rail-preview-btn {
+  display: block;
+  width: 100%;
+  padding: 8px 12px;
+  margin-bottom: 16px;
+  background: var(--accent);
+  color: var(--paper);
+  border: none;
+  border-radius: var(--r3);
+  font-size: 13px;
+  font-weight: 600;
+  cursor: pointer;
+  text-align: center;
+  transition: background 0.12s;
+}
+.rail-preview-btn:hover { background: var(--accent-2); }
+/* Open-in-new-tab button in preview modal header */
+.preview-modal-newtab {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 28px;
+  height: 28px;
+  border-radius: var(--r2);
+  color: var(--muted-2);
+  text-decoration: none;
+  font-size: 16px;
+  flex-shrink: 0;
+  transition: background 0.12s, color 0.12s;
+}
+.preview-modal-newtab:hover { background: var(--field); color: var(--ink); }
+/* Push content above the fixed AudioBar when it is visible */
+body.has-audio-bar { padding-bottom: 56px; }