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/App.tsx

@@ -123,6 +123,9 @@ function Shell({ jobPageSize, setJobPageSize, jobColumns, setJobColumns, themeMo
 
   const path = location.pathname;
   const isJobs = path.startsWith("/jobs");
+  const shortcutHint = useMemo(() => (
+    typeof navigator !== "undefined" && /Mac|iPhone|iPod|iPad/.test(navigator.platform) ? "⌘K" : "Ctrl+K"
+  ), []);
 
   useEffect(() => {
     api.get<AuthConfig>("/auth/config").then((r) => setRequireAuth(Boolean(r.data?.requireAuth))).catch(() => setRequireAuth(false));
@@ -246,14 +249,19 @@ function Shell({ jobPageSize, setJobPageSize, jobColumns, setJobColumns, themeMo
         <IconButton
           color="secondary"
           size="small"
-          title={t("quickSearch")}
+          title={`${t("quickSearch")} (${shortcutHint})`}
           onClick={() => setQuickOpen(true)}
           sx={{ border: "1px solid", borderColor: "divider", borderRadius: 2.5, width: 42, height: 42, flex: "0 0 auto" }}
         >
           <SearchIcon fontSize="small" />
         </IconButton>
       ) : (
-        <Button variant="outlined" startIcon={<SearchIcon />} onClick={() => setQuickOpen(true)}>{t("quickSearch")}</Button>
+        <Button variant="outlined" startIcon={<SearchIcon />} onClick={() => setQuickOpen(true)} sx={{ gap: 0.5 }}>
+          {t("quickSearch")}
+          <Box component="span" sx={{ ml: 0.75, px: 0.75, py: 0.125, borderRadius: 1, border: "1px solid", borderColor: "divider", fontSize: 11, fontWeight: 700, color: "text.secondary", lineHeight: 1.6 }}>
+            {shortcutHint}
+          </Box>
+        </Button>
       )}
       {isJobs ? (
         <Button variant="contained" onClick={() => setAddOpen(true)} sx={{ flex: { xs: 1, sm: "0 0 auto" }, minHeight: 42 }}>
@@ -284,7 +292,7 @@ function Shell({ jobPageSize, setJobPageSize, jobColumns, setJobColumns, themeMo
       >
         <Suspense fallback={<PageLoader />}>
           <Routes>
-            <Route path="/" element={<Navigate to="/jobs" replace />} />
+            <Route path="/" element={<Navigate to="/dashboard" replace />} />
             <Route path="/dashboard" element={<DashboardView />} />
             <Route path="/jobs" element={<JobTable refreshToken={refreshToken} pageSize={jobPageSize} onPageSizeChange={setAndPersistPageSize} columns={jobColumns} onColumnsChange={setAndPersistColumns} mode="jobs" />} />
             <Route path="/reminders" element={<RemindersView />} />