feat(ui): adopt JobTrack mockup design tokens (indigo brand, light dashboard)

First UI slice toward the product mockups. Shifts the shared MUI theme to match
the mockup identity without restructuring components:
- Default accent -> indigo #6366f1 (drives primary buttons, active nav, and the
  Applied/Interview status colours). Users with a custom accent keep theirs.
- Light app background -> #F4F6FB with white paper, giving the layered dashboard
  look (cards/inputs sit above a soft grey canvas).
- Global corner radius 8 -> 10 to match the mockups' rounder cards.
- Accent picker: offer indigo/cyan/violet presets first (kept #15803d).

Behaviour-only theme change. Frontend suite: 38/39 pass; the 1 failure
(end-to-end-trust-loop tailored-CV) is pre-existing on this branch (verified by
running it on the clean tree) and unrelated.

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

job-tracker-ui/src/themePrefs.ts

@@ -23,7 +23,7 @@ export function setThemeModePref(v: ThemeModePref) {
 export function getAccentColor(): string {
   const raw = window.localStorage.getItem(k("accentColor"));
   if (raw && /^#[0-9a-fA-F]{6}$/.test(raw)) return raw;
-  return "#15803d";
+  return "#6366f1";
 }
 
 export function setAccentColor(v: string) {