pdf-viewer: various visual and usability changes

- fix page nav buttons, smaller default pages, violet bg

  - Build toolbar after renderAllPages so it queries existing canvases;
    query canvases fresh in the scroll handler so page tracking works
    after re-render (zoom in/out).
  - Default data-pdf-scale 0.85 (JS fallback 0.8) so pages are a bit
    smaller by default.
  - Add dark transparent violet background (accent-secondary 22%) to
    .pdf-expanded, behind the pages.

  - expand to fill column top-to-bottom via CSS flex

    Use a flex chain (.tfe-files flex:1, .tfe-file-item:has(.pdf-expanded)
    flex:1) so the expanded PDF wrapper fills the right column from under
    the header to the bottom of the viewport, instead of viewport-based
    clamp math that didn't account for the header height.

  - fill right column, center pages, freeze column on expand

  The expanded viewer now stays inside the right column instead of
  growing the page: the column switches to overflow:hidden while the
  PDF panel scrolls internally. Pages are centered horizontally via
  flexbox align-items: center.

  - fix toolbar not full-width, hide sibling file items

  The toolbar was constrained by the parent's align-items:center; fixed
  with align-self:stretch. Hiding sibling .tfe-file-item elements when
  a PDF is expanded prevents them from rendering above the overlay.

  - disable overscroll on expanded view

  - use overscroll-behavior-y on container, none on toolbar

  - disable body overscroll via .pdf-viewer-open class

  - mobile full-viewport, iframe matches pdf height

  - mobile overlay above everything via position:fixed

  - opaque dark-violet bg on mobile overlay

  - tfe: add 'Open in new tab' link above website iframes

  - trim column padding when expanded for more vertical space

  - 5% zoom increments, subtle padding on expanded column

  - prevent horizontal overflow/overscroll

app/public/assets/css/tfe.css

@@ -57,6 +57,8 @@
 	display: flex;
 	flex-direction: column;
 	gap: var(--space-m);
+	flex: 1;
+	min-height: 0;
 }
 
 .tfe-file-item {
@@ -70,6 +72,30 @@
 	overflow: visible;
 }
 
+.tfe-file-item:has(.pdf-expanded) {
+	flex: 1;
+	min-height: 0;
+	overflow: visible;
+}
+
+.tfe-file-item:has(.tfe-file-iframe) {
+	flex: 1;
+	min-height: 0;
+	overflow: visible;
+}
+
+/* When a PDF is expanded, freeze the right column and hide
+   sibling file items so they can't render above the expanded view.
+   Also trim column padding so the PDF fills more vertical space. */
+.tfe-right-column:has(.pdf-expanded) {
+	overflow: hidden;
+	padding-top: var(--space-xs);
+	padding-bottom: var(--space-xs);
+}
+.tfe-right-column:has(.pdf-expanded) .tfe-file-item:not(:has(.pdf-expanded)) {
+	display: none;
+}
+
 .tfe-file-item img {
 	width: 100%;
 	height: auto;
@@ -95,11 +121,29 @@
 .tfe-file-iframe {
 	width: 100%;
 	max-width: 100%;
-	height: clamp(300px, 80vh, 700px);
+	height: 100%;
+	min-height: 400px;
 	display: block;
 	border: none;
 }
 
+.tfe-website-link {
+	display: inline-flex;
+	align-items: center;
+	gap: var(--space-3xs);
+	padding: var(--space-2xs) var(--space-xs);
+	font-size: var(--step--1);
+	color: var(--accent-primary);
+	text-decoration: none;
+	border: 1px solid var(--border);
+	border-radius: var(--radius);
+	margin-bottom: var(--space-2xs);
+}
+
+.tfe-website-link:hover {
+	background: color-mix(in srgb, var(--accent-primary) 10%, var(--bg-primary));
+}
+
 .tfe-file-item figcaption {
 	font-size: var(--step--2);
 	color: var(--text-secondary);
@@ -312,8 +356,8 @@
 	background: var(--surface);
 	border: 1px solid var(--border);
 	border-radius: var(--radius);
-	outline: none;
 	transition: height 0.35s ease, border-color 0.35s ease;
+	outline: none;
 }
 
 /* ── Collapsed state: thumbnail tile ────────── */
@@ -395,19 +439,30 @@
 
 /* ── Expanded state: wrapper becomes scroll panel ── */
 
+/* Disable root-level overscroll (rubberbanding) while a PDF is open. */
+body.pdf-viewer-open {
+	overscroll-behavior: none;
+}
+
 .pdf-expanded {
-	/* Fill viewport minus the column's vertical padding */
-	max-height: calc(100vh - var(--space-xl) * 2);
-	height: clamp(400px, 80vh, calc(100vh - var(--space-xl) * 2));
+	/* Fill the right column from top to bottom (under the header). */
+	height: 100%;
+	min-height: 400px;
 	overflow-y: auto;
+	overflow-x: hidden;
+	overscroll-behavior-y: contain;
 	border: 2px solid var(--accent-primary);
-	display: block;
+	display: flex;
+	flex-direction: column;
+	align-items: center;
+	background: color-mix(in srgb, var(--accent-secondary) 22%, transparent);
 }
 
 /* ── Toolbar ────────────────────────────────── */
 
 .pdf-toolbar {
 	position: sticky;
+	align-self: stretch;
 	top: 0;
 	z-index: 2;
 	display: flex;
@@ -420,6 +475,7 @@
 	flex-shrink: 0;
 	font-size: var(--step--1);
 	user-select: none;
+	overscroll-behavior: none;
 }
 
 .pdf-toolbar-btn {
@@ -505,6 +561,19 @@
 	.tfe-title {
 		font-size: var(--step-2);
 	}
+
+	/* PDF expands to full viewport height on mobile, above everything */
+	.pdf-expanded {
+		position: fixed;
+		inset: 0;
+		z-index: 100;
+		height: auto;
+		min-height: unset;
+		background: color-mix(in srgb, var(--accent-secondary) 92%, #000 8%);
+	}
+	.tfe-file-iframe {
+		height: 100dvh;
+	}
 }
 
 @media (max-width: 600px) {