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/src/icon.php

@@ -0,0 +1,35 @@
+<?php
+
+/**
+ * Inline SVG icon helper.
+ *
+ * Returns the SVG markup for an icon from /assets/icons/{name}.svg.
+ * The SVG is inlined into the DOM so CSS `color` / `fill` cascade naturally.
+ * Width/height are set to 1em by default; override via CSS on the parent.
+ *
+ * Usage: <?= icon('trash') ?>
+ *        <?= icon('search', 0, 'header-search-icon') ?>
+ */
+function icon(string $name, int $size = 0, string $class = ''): string {
+    $path = APP_ROOT . "/public/assets/icons/{$name}.svg";
+    if (!file_exists($path)) {
+        return "<!-- icon not found: {$name} -->";
+    }
+    $svg = file_get_contents($path);
+    // Normalise width/height to 1em so icons scale with font-size
+    $svg = preg_replace('/\bwidth="[^"]*"/', 'width="1em"', $svg);
+    $svg = preg_replace('/\bheight="[^"]*"/', 'height="1em"', $svg);
+    // Ensure aria-hidden by default (icons are decorative when used via this helper)
+    if (!str_contains($svg, 'aria-hidden')) {
+        $svg = str_replace('<svg', '<svg aria-hidden="true"', $svg);
+    }
+    // Inject CSS class if provided
+    if ($class !== '') {
+        if (str_contains($svg, 'class="')) {
+            $svg = str_replace('class="', 'class="' . $class . ' ', $svg);
+        } else {
+            $svg = str_replace('<svg', '<svg class="' . $class . '"', $svg);
+        }
+    }
+    return $svg;
+}