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/admin/actions/peertube-orphans-fragment.php

@@ -20,7 +20,7 @@ if (!($d['configured'] ?? false)): ?>
 <div id="peertube-orphans-wrapper">
     <details id="peertube-orphans-col" class="n-section" open>
         <summary>
-            <img src="/assets/icons/video.svg" width="14" height="14" alt="" aria-hidden="true">
+            <?= icon('video') ?>
             Vidéos PeerTube orphelines
         </summary>
         <div id="peertube-orphans-stats">
@@ -34,7 +34,7 @@ if (!($d['configured'] ?? false)): ?>
 <div id="peertube-orphans-wrapper">
     <details id="peertube-orphans-col" class="n-section" open>
         <summary>
-            <img src="/assets/icons/video.svg" width="14" height="14" alt="" aria-hidden="true">
+            <?= icon('video') ?>
             Vidéos PeerTube orphelines
         </summary>
         <div id="peertube-orphans-stats">
@@ -47,7 +47,7 @@ if (!($d['configured'] ?? false)): ?>
 <div id="peertube-orphans-wrapper">
     <details id="peertube-orphans-col" class="n-section" open>
         <summary>
-            <img src="/assets/icons/video.svg" width="14" height="14" alt="" aria-hidden="true">
+            <?= icon('video') ?>
             Vidéos PeerTube orphelines <span class="n-meta"><?= (int)($d['orphan_count'] ?? 0) ?> vidéos orphelines</span>
         </summary>
         <div id="peertube-orphans-stats">
@@ -70,7 +70,7 @@ if (!($d['configured'] ?? false)): ?>
                                     hx-swap="outerHTML"
                                     hx-trigger="click"
                                     hx-indicator="#peertube-orphans-wrapper">
-                                <img src="/assets/icons/trash.svg" width="14" height="14" alt="" aria-hidden="true">
+                                <?= icon('trash') ?>
                                 Supprimer
                             </button>
                         </td>
@@ -87,7 +87,7 @@ if (!($d['configured'] ?? false)): ?>
 <?php if (($d['stale_count'] ?? 0) > 0): ?>
     <details id="peertube-stale-section" class="n-section" open>
         <summary>
-            <img src="/assets/icons/warning-diamond.svg" width="14" height="14" alt="" aria-hidden="true">
+            <?= icon('warning-diamond') ?>
             Références DB obsolètes <span class="n-meta"><?= $d['stale_count'] ?></span>
         </summary>
         <p style="margin:0 0 var(--space-sm) 0;font-size:0.85em;color:var(--text-secondary)">Ces UUID sont référencés en base de données mais n'existent plus sur la chaîne PeerTube. Les TFE liés affichent des liens morts.</p>