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/JobDetailsDialog.tsx

@@ -1132,6 +1132,7 @@ export default function JobDetailsDialog({ open, jobId, onClose, initialTab = 0,
             <MatchScoreCard score={matchScore} loading={loadingMatchScore} />
             {loadingCandidateFit ? <Box sx={{ py: 4, display: "flex", justifyContent: "center" }}><CircularProgress size={28} /></Box> : candidateFit ? (
               <Box sx={{ display: "flex", flexDirection: "column", gap: 2.5 }}>
+                <Typography variant="caption" sx={{ color: "text.secondary", display: "block", mt: -1 }}>{t("jobDetailsAiFitHint")}</Typography>
                 <Box sx={{ display: "flex", justifyContent: "space-between", alignItems: "center", gap: 1, flexWrap: "wrap" }}>
                   <Box><Typography variant="overline">{t("jobDetailsHowYouMatch")}</Typography><Typography sx={{ whiteSpace: "pre-wrap" }}>{candidateFit.matchSummary}</Typography></Box>
                   <Box sx={{ display: "flex", gap: 1, flexWrap: "wrap", alignItems: "center" }}>