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/icons/caret-right.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="32" height="32" fill="#000000" viewBox="0 0 256 256"><path d="M181.66,133.66l-80,80a8,8,0,0,1-11.32-11.32L164.69,128,90.34,53.66a8,8,0,0,1,11.32-11.32l80,80A8,8,0,0,1,181.66,133.66Z"></path></svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="32" height="32" fill="currentColor" viewBox="0 0 256 256"><path d="M181.66,133.66l-80,80a8,8,0,0,1-11.32-11.32L164.69,128,90.34,53.66a8,8,0,0,1,11.32-11.32l80,80A8,8,0,0,1,181.66,133.66Z"></path></svg>