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/plugins/tests/tests.js

@@ -55,6 +55,7 @@ const FILES = [
     './terminal.test.js',
     './theme.test.js',
     './chat.test.js',
+    './shopping.test.js',
 ];
 
 // Await all imports so the runner can start in a deterministic state.