style(ui): float dialogs/menus/popovers, round chips and tooltips globally

Every Dialog/Menu/Popover in the app renders its content via MuiPaper,
which is deliberately kept flat (1px border, no shadow -- Paper is
used too broadly, e.g. as a plain content divider, to safely restyle
globally). That meant every modal (confirm/prompt, AddJobModal,
EditJobDialog, JobDetailsDialog) and every dropdown/Select menu in the
app was still rendering flat-bordered despite every other screen this
session moving to the floating-shadow mockup look.

Added targeted MuiDialog/MuiPopover/MuiMenu paper overrides -- these
win on specificity over MuiPaper's own defaults without touching
MuiPaper itself, so every dialog and dropdown in the app picks up the
rounded floating-shadow treatment from this one change instead of
patching each dialog file individually. Also added MuiChip (full pill
radius, matching every status/skill pill in the mockups) and
MuiTooltip (matching corner radius) overrides, and gave the toast
Snackbar/Alert a consistent radius + weight.

job-tracker-ui/src/toast.tsx

@@ -53,7 +53,7 @@ export function ToastProvider({ children }: { children: React.ReactNode }) {
           onClose={() => setT((p) => ({ ...p, open: false }))}
           severity={t.severity}
           variant="filled"
-          sx={{ maxWidth: 420 }}
+          sx={{ maxWidth: 420, borderRadius: 3, fontWeight: 600 }}
           action={
             t.actionLabel && t.onAction ? (
               <Button