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/layout/AppShell.tsx

@@ -59,6 +59,7 @@ const SIDEBAR_SELECTED_ICON = "#a5b4fc";
 
 export default function AppShell({
   pageTitle,
+  pageSubtitle,
   breadcrumbs,
   pathname,
   nav,
@@ -76,6 +77,7 @@ export default function AppShell({
   children,
 }: {
   pageTitle: string;
+  pageSubtitle?: string;
   breadcrumbs: string[];
   pathname: string;
   nav: NavItem[];
@@ -481,6 +483,11 @@ export default function AppShell({
               <Typography variant="h5" sx={{ fontWeight: 600, overflowWrap: "anywhere" }}>
                 {pageTitle}
               </Typography>
+              {pageSubtitle ? (
+                <Typography variant="body2" sx={{ color: "text.secondary", mt: 0.25, overflowWrap: "anywhere" }}>
+                  {pageSubtitle}
+                </Typography>
+              ) : null}
             </Box>
           </Box>