relay UI overhaul, favicon, settings, admin last-seen, fixes

Relay:
  - Inbox/shared/sent split into own view-inbox (nav button + poll).
    Compose is now single-column, centered.
  - Header logo links to compose. Inbox badge stays fresh on all
    views via a quiet loadInbox on every view switch.
  - Mobile menu: full-height sheet below header, larger buttons
    (15px font, 14px padding), full width.
  - Login form wrapped in <form> with name attributes + for/id on
    labels so password managers can autofill.
  - Admin user list shows "last seen <relative time>" per user.
    last_seen_at column on tenants, updated on every authenticated
    request (session + client API).
  - Cache-busting on style.css + app.js via filemtime.
  - View persistence in localStorage (survives reload).
  - Shared-bucket cards show title only (preview line removed).

Favicon:
  - favicon.svg added to project root + copied to public/, relay/,
    website/. Linked via <link rel="icon" type="image/svg+xml"> in
    all three <head> sections.

Settings:
  - New toggle: "Show shopping list items in task collector"
    (default off). collectTasks skips #einkauf/#shopping cards when
    disabled.

Fixes:
  - card-spinner gets display:inline-block so it rotates outside
    flex containers (shopping save spinner).
  - Tim tab test opens the plugin before checking active state.
  - Compose view centered with margin:auto on desktop.

Bumps to 1.9.29.

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

relay/inc/bootstrap.php

@@ -133,6 +133,14 @@ $db->exec('CREATE TABLE IF NOT EXISTS tenants (
     created_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP
 )');
 
+// last_seen_at: updated on every authenticated request so the admin
+// can see when a user was last active.
+$tenantCols = $db->query("PRAGMA table_info(tenants)")->fetchAll();
+$tenantColNames = array_column($tenantCols, 'name');
+if (!in_array('last_seen_at', $tenantColNames)) {
+    $db->exec("ALTER TABLE tenants ADD COLUMN last_seen_at TEXT");
+}
+
 $db->exec('CREATE TABLE IF NOT EXISTS settings (
     key TEXT PRIMARY KEY,
     value TEXT