Migrate all <img>-based icons to inline SVG via PHP helper

Replace every <img src="/assets/icons/..."> with <?= icon('name') ?>
across 26 template files. The PHP helper inlines the SVG markup into the
DOM so CSS color cascades naturally through fill="currentColor".

- Add src/icon.php helper: reads SVG file, sets width/height to 1em,
  injects aria-hidden, supports optional CSS class
- Fix 12 icon SVGs that had hardcoded fill="#000000" or missing fill attr
- Replace search.svg with Phosphor fill-based magnifying glass
- Add explicit SVG sizes for admin header nav icons (16px/20px)
- Scope public search icon CSS to form[role=search]:not(.header-search-form)
  to avoid breaking admin header layout; change stroke to fill
- Remove <img> filter: brightness(0) invert(1) hacks from admin.css

app/public/assets/css/admin.css

@@ -30,6 +30,8 @@
 }
 .admin-body header nav ul [data-nav-logout] a svg {
     display: block;
+    width: 20px;
+    height: 20px;
 }
 
 /* Public-site link icon in admin nav */
@@ -40,6 +42,11 @@
     column-gap: 0.4em;
 }
 
+.admin-body header nav .nav-logo svg {
+    width: 16px;
+    height: 16px;
+}
+
 
 .admin-body main {
     flex: 1;