feat(ui): dark navy sidebar + restrained kanban status colors

First pass of the /frontend-design overhaul against the mockups at
F:\Pictures\website\jobtracker\new. Two highest-leverage gaps from the
backlog note ("dark sidebar, KPI cards, exact status colours"):

- AppShell: nav rail is now a fixed dark navy (#0f172a) regardless of
  the app's light/dark theme toggle, matching the mockup's signature
  look -- selected item gets an indigo-tinted pill + icon accent,
  muted slate text for the rest. Kept icon+label rows (mockup's sidebar
  is text-only) since the existing collapsed-sidebar mode depends on
  icons; that's a deliberate deviation, not an oversight.
- JobbjaktMark: replaced the briefcase glyph with the gradient
  checkmark-in-square mark used throughout the mockups (hero, dashboard,
  kanban) -- also fixed a latent SVG gradient id collision across
  multiple rendered instances via useId().
- KanbanBoard: mockup uses color sparingly (a small dot in the column
  header, a 4px accent on the card's left edge) rather than tinting the
  whole column/card background as the previous version did. Reworked
  to match; also swapped card title/subtitle order (job title bold,
  company/location as subtitle) per the mockup.

Remaining for follow-up passes: Dashboard KPI card layout and the job
workspace (candidate-fit ring, AI summary card) -- both structurally
close already but not yet pixel-matched.

Verified: `next build` clean, all 57 frontend tests green, dark
sidebar confirmed live (computed bg #0f172a) against a running dev
server with light content mode forced.

job-tracker-ui/src/assets/JobbjaktMark.tsx

@@ -1,21 +1,18 @@
-import React from "react";
+import React, { useId } from "react";
 
 export default function JobbjaktMark(props: React.SVGProps<SVGSVGElement>) {
+  const gradientId = useId();
+
   return (
-    <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" role="img" aria-label="Job tracker" {...props}>
+    <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 34 34" role="img" aria-label="Jobbjakt" {...props}>
       <defs>
-        <linearGradient id="briefcase-track" x1="0" x2="1" y1="0" y2="1">
-          <stop offset="0%" stopColor="#3b82f6" />
-          <stop offset="100%" stopColor="#14b8a6" />
+        <linearGradient id={gradientId} x1="0" x2="1" y1="0" y2="1">
+          <stop offset="0%" stopColor="#6366f1" />
+          <stop offset="100%" stopColor="#22d3ee" />
         </linearGradient>
       </defs>
-      <rect x="8" y="12" width="48" height="40" rx="12" fill="#0f172a" />
-      <path d="M22 20v-2c0-3.3 2.7-6 6-6h8c3.3 0 6 2.7 6 6v2" fill="none" stroke="url(#briefcase-track)" strokeWidth="4" strokeLinecap="round" />
-      <rect x="14" y="22" width="36" height="26" rx="8" fill="none" stroke="url(#briefcase-track)" strokeWidth="4" />
-      <path d="M14 31h14" stroke="url(#briefcase-track)" strokeWidth="4" strokeLinecap="round" />
-      <path d="M36 31h14" stroke="url(#briefcase-track)" strokeWidth="4" strokeLinecap="round" />
-      <circle cx="32" cy="31" r="4.5" fill="#e2e8f0" />
-      <path d="M24 40l5 5 11-12" fill="none" stroke="#e2e8f0" strokeWidth="4" strokeLinecap="round" strokeLinejoin="round" />
+      <rect width="34" height="34" rx="9" fill={`url(#${gradientId})`} />
+      <path d="M9 17.5l5 5 11-12" fill="none" stroke="#ffffff" strokeWidth="3.4" strokeLinecap="round" strokeLinejoin="round" />
     </svg>
   );
 }