viewer: Phase 0 redesign — slim headers, navy accent, icon nav

First pass of the viewer redesign (see docs/review/VIEWER_REDESIGN.md), no
structural change:

- TitleLabel: replace the 50px solid #FA7268 section bars with a slim 26px
  navy-bold header + coral accent rule, removing the "venetian blind" stack
  while keeping the salmon identity.
- Palette: switch the accent (QPalette::Highlight) from teal to navy #1F3A5F.
- Image toolbar: replace the unicode arrow glyphs (|<= <= => =>|) with flat
  media icons (first/prev/next/last) + tooltips, and a play icon on Movie.
- Bottom dock: give the per-dataset plot more default height (340 -> 420).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

viewer/widgets/TitleLabel.cpp

@@ -4,8 +4,18 @@
 #include "TitleLabel.h"
 
 TitleLabel::TitleLabel(QString text, QWidget *parent) : QLabel(parent) {
-    setText(QString("<h3>%1</h3>").arg(text));
-    setStyleSheet("background-color: #FA7268; color: #ffffff;");
-    setAlignment(Qt::AlignmentFlag::AlignCenter);
-    setFixedHeight(50);
+    // Slim section header: navy bold text with a coral accent rule, in place of a heavy 50px
+    // full-width salmon bar. Keeps the salmon identity (the rule) while letting panels breathe.
+    setText(text);
+    QFont f = font();
+    f.setBold(true);
+    setFont(f);
+    setStyleSheet("QLabel {"
+                  " color: #1F3A5F;"
+                  " background-color: transparent;"
+                  " border-left: 3px solid #FA7268;"
+                  " border-bottom: 1px solid #FA7268;"
+                  " padding: 3px 6px 3px 8px; }");
+    setAlignment(Qt::AlignLeft | Qt::AlignVCenter);
+    setFixedHeight(26);
 }