shopping: #shopping tag, help section, tests, client-side proxy

Three follow-ups:

  - Accept #shopping as an alias for #einkauf. Either tag now makes
    a card show up in the picker; the help blurb in the dialog says
    so explicitly.
  - Add a proper help-overlay section covering the trigger tags,
    how to pick an active list, the phone URL, sync cadence, and
    why phone is check-only.
  - Add a client-side proxy in public/index.php for /api/shopping
    and /api/shopping/check — shopping.js was calling api('api/...')
    which went to hyprcrd, but only the relay had handlers. Without
    this forwarder, push/pull just 404'd. Proxy mirrors the pattern
    used by /api/relay/push.
  - New shopping.test.js: asserts the proxy returns a JSON shape
    with `ok` (works whether or not a relay is configured), the
    hamburger menu toggles as expected, and the picker lists both
    #einkauf + #shopping cards while excluding untagged ones.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/js/modules/shopping.js

@@ -19,7 +19,8 @@ const LAST_PUSHED_KEY = 'hyprcrd-shopping-last-pushed-ts';
 const LAST_SERVER_KEY = 'hyprcrd-shopping-last-server-ts';
 
 function hasEinkaufTag(card) {
-    return (card.tags || '').split(',').map(s => s.trim()).includes('einkauf');
+    const tags = (card.tags || '').split(',').map(s => s.trim());
+    return tags.includes('einkauf') || tags.includes('shopping');
 }
 
 function getActiveId() {
@@ -59,9 +60,10 @@ function openDialog() {
             <button class="shopping-picker-close text-t-muted hover:text-t-danger"><i class="ph ph-x text-sm"></i></button>
         </div>
         <div class="px-5 py-3 text-[11px] text-t-muted leading-relaxed">
-            Karten mit dem Tag <span class="text-t-accent">#einkauf</span> können als aktive Liste
-            fürs Telefon gewählt werden. Nur eine ist aktiv; ihr Body wird bei jedem Sync zum
-            Relay geschickt und unter <span class="text-t-accent">/shopping</span> angezeigt.
+            Karten mit Tag <span class="text-t-accent">#einkauf</span> oder
+            <span class="text-t-accent">#shopping</span> können als aktive Liste fürs Telefon
+            gewählt werden. Nur eine ist aktiv; ihr Body wird bei jedem Sync zum Relay geschickt
+            und unter <span class="text-t-accent">/shopping</span> angezeigt.
         </div>
         <div class="flex-1 overflow-y-auto px-2 py-2" id="shopping-picker-list"></div>
         <div class="flex items-center gap-2 px-5 py-3 border-t border-t-border">