shopping: ship as plugin, integrate phone view into relay UI, fix sync

Restructure the shopping-list feature end to end:

  - New public/plugins/shopping/ plugin with backend.php (proxy
    routes registered via PluginManager::onApiRoute), shopping.js
    (self-contained; injects the header Shopping button, owns the
    picker dialog + sync loop via hyprcrd.hooks), plugin.json and a
    placeholder css file. Core no longer knows about shopping —
    index.php loses the static button and the proxy block, app.js
    loses the initShopping wiring, the old js/modules/shopping.js
    is gone.
  - Phone view moves out of a standalone /shopping page and into
    the relay UI as a regular `view-shopping` behind a nav-burger
    entry (relay/ui/layout.php + app.js + style.css). Same session
    auth as compose/chat/account — no separate login. Auto-refresh
    every 4 s while the view is active; no manual refresh button.
    Headings "# h1 … ###### h6" render between items for grouping.
    Orphan section headings (no open items under them) are hidden.
  - Sync: drop timestamp cursors (broke across SQLite-UTC / PHP
    timezones and silently blocked pushes after a pull). Both sides
    diff the (uid, title, body) triple instead — timezone-agnostic
    and authoritative. Client ticks push-before-pull so fresh local
    edits land on the relay before a stale pull can overwrite them.
    Relay drops its "server newer" guard and stamps server times
    with gmdate so anything left comparing to SQLite is UTC.
  - Client also ticks on cards.reloaded, not just sync.tick, so
    any edit path (editor save, checkbox toggle, context menu,
    undo) pushes the update immediately.
  - Tests: updated for the plugin (still uses the same selectors
    because the plugin injects the same ids), plus a plugin-
    registration check and "(no active list)" option assertion.
  - Help section copy updated — references the Shopping header
    button and the shopping tab in the relay UI rather than the
    gone /shopping URL.

Both #einkauf and #shopping work as trigger tags; phone is
check-only, editing stays in hyprcrd. Version bumped to 1.9.21.

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

public/js/app.js

@@ -21,7 +21,6 @@ import { openCalendar, closeCalendar, refreshCalendar } from './modules/calendar
 import { loadSettings as loadCaldavSettings } from './modules/caldav.js';
 import { startBackgroundSync, triggerSync, loadRelaySettings } from './modules/sync.js';
 import { openPluginSettings } from './modules/plugin-settings.js';
-import { initShopping } from './modules/shopping.js';
 
 // ── Global API (for inline onclick handlers and cross-module calls) ──
 
@@ -218,7 +217,6 @@ async function checkAuth() {
     loadBuckets();
     loadCards();
     startBackgroundSync();
-    initShopping();
 }
 
 // ── Keyboard shortcuts (central registry) ──