feat(ux): content-shaped route skeletons + disable Cancel during submit

Per-route loading.tsx for dashboard/members/payments (content-shaped
Skeletons instead of the single global full-page spinner) so heavy
server pages paint structure immediately. Add/Edit member + Log cash
modals now disable Cancel while the submit is in flight (was clickable
mid-insert).

app/(app)/payments/payments-client.tsx

@@ -305,7 +305,7 @@ export function PaymentsClient({ payments: initialPayments, members, integration
               </div>
               {error && <p className="font-mono text-xs text-red-500">{error}</p>}
               <div className="flex gap-3 pt-2">
-                <button type="button" onClick={() => setShowLogCash(false)} className="flex-1 border border-border text-foreground font-sans text-sm py-2 rounded hover:border-primary/50 transition">Cancel</button>
+                <button type="button" disabled={loading} onClick={() => setShowLogCash(false)} className="flex-1 border border-border text-foreground font-sans text-sm py-2 rounded hover:border-primary/50 transition disabled:opacity-60">Cancel</button>
                 <button type="submit" disabled={loading} className="flex-1 bg-primary text-white font-sans text-sm py-2 rounded hover:bg-primary/90 transition disabled:opacity-60">
                   {loading ? 'Logging...' : 'Log Payment'}
                 </button>