refactor(aicoc): drop iframe embed path; fix slack icon

Round-2 review feedback from @rofe on #93:

1. Recording is always a link, never an iframe embed.
   Removed EMBEDDABLE_HOSTS allowlist and the iframe-building branch in
   buildRecording(). Recordings now always render as a "Watch recording"
   button (or "Watch on SharePoint" when the URL is on sharepoint.com).
   Also removed the .session-header-embed CSS that supported the iframe.

2. Replaced icons/slack.svg with the official Slack monochrome logo.
   The previous path was malformed; the new one is the recognisable
   4-element hash shape.

Both lint:js and lint:css continue to pass clean.

Also cleaned up an iCloud sync-conflict duplicate `styles/aicoc 2.css`
that was tripping stylelint with stale content from before the modern
color-function migration.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: Dominik Steinacher

blocks/session-header/session-header.css

@@ -239,24 +239,6 @@ main .session-header .btn-tertiary {
   border-color: rgb(255 255 255 / 30%);
 }
 
-/* Inline iframe embed (Stream / YouTube / Vimeo only) */
-.session-header-embed {
-  position: relative;
-  margin-top: 24px;
-  width: 100%;
-  max-width: 720px;
-  aspect-ratio: 16 / 9;
-  border-radius: 12px;
-  overflow: hidden;
-  box-shadow: 0 12px 40px rgb(0 0 0 / 35%);
-}
-
-.session-header-embed iframe {
-  width: 100%;
-  height: 100%;
-  border: 0;
-}
-
 /* ── SIDEBAR ──────────────────────────────────────────────────────────── */
 .session-header-sidebar {
   padding: 28px 32px 8px;

blocks/session-header/session-header.js

@@ -1,17 +1,5 @@
 import { getMetadata, createOptimizedPicture, decorateIcons } from '../../scripts/lib-franklin.js';
 
-/**
- * Hosts whose URLs can safely be iframe-embedded as a video player.
- * Anything else (SharePoint, intranet, unknown) renders as a "Watch recording"
- * button that opens in a new tab.
- */
-const EMBEDDABLE_HOSTS = new Set([
-  'web.microsoftstream.com',
-  'youtube.com',
-  'www.youtube.com',
-  'youtu.be',
-]);
-
 // Icon markers — replaced with inline SVG by decorateIcons() at end of decorate.
 // SVG sources live in /icons/<name>.svg.
 const icon = (name) => `<span class="icon icon-${name}"></span>`;
@@ -65,30 +53,18 @@ function splitCommaList(raw) {
 }
 
 /**
- * Build the recording widget. SharePoint and unknown hosts render as a
- * "Watch recording" button (opens in a new tab). Public Stream / YouTube
- * URLs become an inline iframe.
+ * Build the recording button — opens the recording in a new tab.
+ * SharePoint URLs get a "Watch on SharePoint" label; everything else gets
+ * "Watch recording".
  */
 function buildRecording(url) {
   if (!url) return null;
   let host = '';
-  try { host = new URL(url).hostname.toLowerCase(); } catch (_) { /* malformed */ }
-
-  if (EMBEDDABLE_HOSTS.has(host)) {
-    return el(
-      'div',
-      { class: 'session-header-embed' },
-      el('iframe', {
-        src: url,
-        title: 'Session recording',
-        allowFullscreen: true,
-        frameBorder: '0',
-        allow: 'autoplay; encrypted-media; picture-in-picture',
-      }),
-    );
+  try {
+    host = new URL(url).hostname.toLowerCase();
+  } catch (_) {
+    // Malformed URL — fall through with empty host; we still render the link.
   }
-
-  // Default: SharePoint or unknown → button (opens in a new tab)
   const label = host.includes('sharepoint.com') ? 'Watch on SharePoint' : 'Watch recording';
   return el('a', {
     class: 'btn btn-primary',