feat(ui): establish premium design foundation from mockups

Extract design tokens from the mockup set (F:\Pictures\website\jobtracker\new
dashboard, pipeline, job-workspace, features, workflow screens) into the
central theme so every screen picks the change up automatically:

- Heading weight: h1-h4 go bold/black (800/700) to match the mockups' heavy
  display type; h5/h6 stay a lighter semibold so dense screens don't turn
  into a wall of black text.
- Card shadow: replace the flat 1px "section" shadow + visible border with a
  soft floating shadow and no border, matching how mockup cards sit on the
  grey page background.
- Border radius: 10/12/8px -> 14/16/10px across shape/card/button defaults,
  matching the mockups' rounder corners.
- New GradientButton component wrapping the mockup's signature indigo->cyan
  CTA gradient ("Tailor my CV for this role", "See the interface tour"),
  reserved for the single most important AI-assist/hero action per screen.

The dark navy sidebar (#0f172a) already matched the mockups from an earlier
pass -- untouched here.

Verified: tsc clean, full frontend suite green (65/65). Live visual
screenshot verification wasn't possible -- the Browser pane's screenshot
tool times out in this environment; verified structurally via read_page
and the app rendering without console errors instead.

job-tracker-ui/src/components/GradientButton.tsx

@@ -0,0 +1,33 @@
+import React from "react";
+
+import Button, { ButtonProps } from "@mui/material/Button";
+
+import { GRADIENT_CTA } from "../theme";
+
+/**
+ * The mockup's gradient AI/primary-action pill ("Tailor my CV for this role", "See the
+ * interface tour"). Reserve this for the single most important AI-assist or hero action on a
+ * screen -- everything else stays a plain contained/outlined Button, or the gradient stops
+ * reading as a signal and becomes wallpaper.
+ */
+export default function GradientButton({ sx, disabled, ...props }: ButtonProps) {
+  return (
+    <Button
+      variant="contained"
+      disableElevation
+      disabled={disabled}
+      sx={[
+        (theme: any) => ({
+          background: disabled ? undefined : GRADIENT_CTA,
+          color: disabled ? undefined : "#fff",
+          boxShadow: disabled ? undefined : theme.vars.customShadows.gradientButton,
+          "&:hover": disabled
+            ? undefined
+            : { background: GRADIENT_CTA, filter: "brightness(1.06)", boxShadow: theme.vars.customShadows.gradientButton },
+        }),
+        ...(Array.isArray(sx) ? sx : sx ? [sx] : []),
+      ]}
+      {...props}
+    />
+  );
+}