feat(client): add the Nostalgia appearance style

Add a Style setting beside mode, accent, radius and background. Nostalgia restyles the app through
tokens only (neutral surface palettes, system type, flat shell, top glow) and pins background,
opacity and brightness. Add a top glow reach slider for both styles. Tidy the theme layer first:
charts read live tokens, bare `rounded` follows the radius scale, one shared sign-in toolbar.
Also carries the settings page header redesign.

client/src/locales/en.json

@@ -107,10 +107,52 @@
       },
       "theme": {
         "title": "Theme",
+        "style": {
+          "label": "Style",
+          "hint": "The overall look of the app. Mode, accent, corners and background keep their settings.",
+          "bookorbit": {
+            "label": "BookOrbit",
+            "description": "Tinted surfaces, glass panels and serif headings."
+          },
+          "nostalgia": {
+            "label": "Nostalgia",
+            "description": "Neutral grounds, system type, flat panels and a top glow."
+          }
+        },
         "colorScheme": {
           "label": "Color scheme",
           "hint": "Light or dark interface"
         },
+        "surfacePalette": {
+          "label": "Surface",
+          "hint": "The neutral ground behind panels and cards.",
+          "palettes": {
+            "onyx": "Onyx",
+            "carbon": "Carbon",
+            "neutral": "Neutral",
+            "zinc": "Zinc",
+            "iron": "Iron",
+            "steel": "Steel",
+            "ash": "Ash",
+            "smoke": "Smoke",
+            "charcoal": "Charcoal",
+            "slate": "Slate",
+            "midnight-blue": "Midnight blue",
+            "ocean": "Ocean",
+            "pine": "Pine",
+            "olive": "Olive",
+            "stone": "Stone",
+            "sepia": "Sepia",
+            "crimson": "Crimson",
+            "plum": "Plum"
+          }
+        },
+        "topGlow": {
+          "label": "Top glow",
+          "hint": "How far the accent glow reaches down the page. Slide all the way left to turn it off.",
+          "value": "{value}%",
+          "off": "Off"
+        },
         "accents": {
           "grey": "Grey",
           "scarlet": "Scarlet",
@@ -183,7 +225,13 @@
         },
         "cornerRadius": {
           "label": "Corner radius",
-          "hint": "Roundness of cards and UI elements"
+          "hint": "Roundness of cards and UI elements",
+          "options": {
+            "sharp": "Sharp",
+            "default": "Default",
+            "rounded": "Rounded",
+            "pill": "Pill"
+          }
         },
         "surfaceBrightness": {
           "label": "Surface brightness",
@@ -202,7 +250,8 @@
           "structural": "Structural",
           "ambient": "Ambient",
           "refractive": "Refractive"
-        }
+        },
+        "syncSaveFailed": "Failed to save theme preferences"
       },
       "storage": {
         "title": "Where to save appearance preferences",
@@ -3615,6 +3664,7 @@
     "nav": {
       "title": "Settings",
       "ariaLabel": "Settings sections",
+      "breadcrumbLabel": "Breadcrumb",
       "searchPlaceholder": "Search settings",
       "clearSearch": "Clear search",
       "noResults": "No settings match {query}.",
@@ -7916,6 +7966,9 @@
       "uploadBooks": "Upload books",
       "appearance": "Appearance",
       "appearanceDescription": "Choose theme, accent, corner radius, surface, and background options.",
+      "style": "Style",
+      "surfacePalette": "Surface",
+      "topGlow": "Top glow",
       "theme": "Theme",
       "accent": "Accent",
       "radius": "Radius",
@@ -8213,6 +8266,28 @@
       "discardTitle": "Discard changes?",
       "discardDescription": "Your unsaved changes will be lost.",
       "discardConfirm": "Discard"
+    },
+    "backgroundPicker": {
+      "none": "None",
+      "dots": "Dots",
+      "cross": "Cross",
+      "millimeter": "Millimeter",
+      "blueprint": "Blueprint",
+      "brushed": "Brushed",
+      "scanlines": "Scanlines",
+      "vinyl": "Vinyl",
+      "carbon": "Carbon",
+      "perforated": "Perforated",
+      "aurora": "Aurora",
+      "horizon": "Horizon",
+      "glow": "Glow",
+      "mesh": "Mesh",
+      "elevation": "Elevation",
+      "prism": "Prism",
+      "spectrum": "Spectrum",
+      "spectrum-x": "Spectrum X",
+      "spectrum-plus": "Spectrum Plus",
+      "eclipse": "Eclipse"
     }
   },
   "customIcons": {