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;

app/public/assets/css/components/search.css

@@ -3,6 +3,21 @@
    Root class: .header-search-wrap
    ============================================================ */
 
+/* Public search bar icon (in partials/search-bar.php) */
+/* Scoped to forms that are NOT inside .header-search-wrap */
+form[role="search"]:not(.header-search-form) > svg {
+  width: 20px;
+  height: 20px;
+  flex-shrink: 0;
+  fill: var(--text-tertiary);
+  margin-right: var(--space-2xs);
+}
+
+form[role="search"]:not(.header-search-form) {
+  display: flex;
+  align-items: center;
+}
+
 .header-search-wrap {
   padding: 0;
   flex-shrink: 0;
@@ -23,7 +38,7 @@
   left: var(--space-s);
   width: 18px;
   height: 18px;
-  stroke: var(--accent-primary);
+  fill: var(--accent-primary);
   pointer-events: none;
 }