feat(ux): first-time onboarding, empty states, and copy fixes

Implements the six "propose first" items from the product/UX review:

- "/" now redirects to /dashboard instead of the empty /jobs table --
  a new user's first screen is now an overview with orientation, not
  a data table with zero rows and four filter dropdowns.
- Jobs table gets a real first-time empty state (distinct from "no
  results match your filters") pointing at Add Job and the bookmarklet,
  instead of a bare "No jobs found."
- Match Score card and Candidate Fit tab now each get a one-line
  caption explaining what they are and how they differ (deterministic
  keyword coverage vs. AI opinion) -- they previously sat side by side
  with no explanation of why there are two.
- Google sign-in hint now reflects self-serve signup when
  Auth:AllowRegistration is on, instead of always implying you need an
  existing linked account.
- Quick Search button now shows its keyboard shortcut (Ctrl+K / ⌘K)
  inline instead of being undiscoverable.

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

@@ -51,6 +51,7 @@ export default function GoogleAuthCard({ onSignedIn }: { onSignedIn?: () => void
   const { t } = useI18n();
   const [me, setMe] = useState<MeResponse | null>(null);
   const [working, setWorking] = useState(false);
+  const [allowRegistration, setAllowRegistration] = useState(false);
   const hostRef = useRef<HTMLDivElement | null>(null);
 
   const clientId = (process.env.NEXT_PUBLIC_GOOGLE_CLIENT_ID || "").trim();
@@ -72,6 +73,9 @@ export default function GoogleAuthCard({ onSignedIn }: { onSignedIn?: () => void
 
   useEffect(() => {
     void refreshMe();
+    api.get<{ allowRegistration: boolean }>("/auth/config").then((res) => {
+      setAllowRegistration(Boolean(res.data?.allowRegistration));
+    }).catch(() => setAllowRegistration(false));
   }, []);
 
   useEffect(() => {
@@ -156,7 +160,7 @@ export default function GoogleAuthCard({ onSignedIn }: { onSignedIn?: () => void
 
           {!signedIn ? (
             <Typography sx={{ color: "text.secondary" }}>
-              {t("googleSignInHint")}
+              {allowRegistration ? t("googleSignInHintSelfServe") : t("googleSignInHint")}
             </Typography>
           ) : me?.provider === "local" ? (
             <Typography sx={{ color: "text.secondary" }}>