style(ui): redesign error pages and job table empty/loading state

No mockup exists for 404/500 pages, so these follow the visual
language already established elsewhere this session: floating-shadow
card, big bold status number (matching the dashboard stat tiles'
bold-number treatment) instead of a small overline.

JobTable's first-run empty state gets an icon chip matching the
landing page's feature-card icon treatment (rounded square, tinted
primary background) instead of plain text -- the filtered "no results"
one-liner stays as-is, that's a different, correctly minimal case.
Main table Paper wrapper gets the same floating-shadow treatment as
every other card this session.

ViewStateNotice (the shared loading/error component used across the
app) reviewed and left untouched -- it's an MUI Alert used as an inline
banner, which is the correct pattern; it was never a "fake card" to
begin with.

job-tracker-ui/src/views/NotFoundPage.tsx

@@ -8,9 +8,16 @@ export default function NotFoundPage() {
   const { t } = useI18n();
 
   return (
-    <Paper sx={{ p: { xs: 3, md: 5 }, borderRadius: 4 }}>
+    <Paper
+      sx={{
+        p: { xs: 4, md: 6 },
+        borderRadius: 4,
+        border: "none",
+        boxShadow: "0px 1px 2px 0px rgba(15,23,42,0.04), 0px 8px 24px -12px rgba(15,23,42,0.12)",
+      }}
+    >
       <Box sx={{ display: "grid", gap: 1.5, maxWidth: 560 }}>
-        <Typography variant="overline" sx={{ color: "text.secondary", letterSpacing: 1.6 }}>
+        <Typography sx={{ fontWeight: 900, fontSize: { xs: 48, md: 64 }, lineHeight: 1, letterSpacing: "-0.02em", color: "primary.main" }}>
           404
         </Typography>
         <Typography variant="h4" sx={{ fontWeight: 800 }}>

job-tracker-ui/src/views/RouteErrorPage.tsx

@@ -16,9 +16,18 @@ export default function RouteErrorPage() {
 
   return (
     <Box sx={{ minHeight: "100vh", display: "grid", placeItems: "center", p: 3 }}>
-      <Paper sx={{ p: { xs: 3, md: 5 }, borderRadius: 4, width: "100%", maxWidth: 640 }}>
+      <Paper
+        sx={{
+          p: { xs: 4, md: 6 },
+          borderRadius: 4,
+          border: "none",
+          boxShadow: "0px 1px 2px 0px rgba(15,23,42,0.04), 0px 8px 24px -12px rgba(15,23,42,0.12)",
+          width: "100%",
+          maxWidth: 640,
+        }}
+      >
         <Box sx={{ display: "grid", gap: 1.5 }}>
-          <Typography variant="overline" sx={{ color: "text.secondary", letterSpacing: 1.6 }}>
+          <Typography sx={{ fontWeight: 900, fontSize: { xs: 48, md: 64 }, lineHeight: 1, letterSpacing: "-0.02em", color: "error.main" }}>
             {error?.status || 500}
           </Typography>
           <Typography variant="h4" sx={{ fontWeight: 800 }}>