feat(ux): per-view subtitles, correspondence cross-links, mobile kanban, a11y

Continuing the product/UX review's deferred items:

- Every top-level view now gets a one-line subtitle under its title
  (Dashboard/Jobs/Kanban/Reminders/Correspondence/Gmail review) stating
  what that specific view is for, instead of navigation being the only
  signal of what each page does.
- Correspondence inbox and Gmail review queue cross-link to each other
  instead of being two unexplained flat sidebar items -- kept both nav
  entries (renaming/nesting risked breaking muscle memory) but made the
  relationship between them explicit in the UI itself.
- Kanban board switches to a horizontal scroll-snap row on phone-width
  viewports instead of stacking all 5 columns vertically, which meant
  a lot of scrolling to see anything past "Applied".
- Match-score ring gets an aria-label with the actual percentage --
  it was two nested decorative CircularProgress elements with no
  accessible text. (Keyboard-accessible status changes on kanban cards
  were already covered by the existing "..." menu -- no gap there.)

job-tracker-ui/src/views/GmailReviewPage.tsx

@@ -138,6 +138,7 @@ export default function GmailReviewPage() {
           <Button variant="outlined" onClick={() => void load()} disabled={loading || syncing}>
             {loading ? "Loading..." : "Refresh"}
           </Button>
+          <Button variant="text" onClick={() => navigate("/correspondence")}>Back to inbox</Button>
         </Box>
       </Box>