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/bootstrap.php

@@ -40,6 +40,7 @@ if (php_sapi_name() === 'cli-server') {
 
 // Central application helper (boot, auth guard, CSRF, flash, render)
 require_once APP_ROOT . '/src/App.php';
+require_once APP_ROOT . '/src/icon.php';
 
 // Maintenance mode gate — block public pages; allow /admin/ through.
 // The flag file lives in storage/ (outside webroot) to avoid web exposure.