feat(search): /search?q= across videos, channels and playlists

Add a dedicated SearchController that matches the query against video
title / description / tags / primary language AND any secondary audio
track language (so "arabic" surfaces videos that carry an Arabic track
even if the primary language is English), plus channel name / username
/ bio, plus playlist name / description. Public visibility only.

Results reuse the videos/index view so every card gets the same
component the browse feeds use. A new Channels section renders above
the Videos grid when channel hits exist.

/videos/search is kept as a 301 redirect to /search so bookmarks and
external links stay valid; both the desktop header and mobile overlay
search forms now point at the canonical route, both carry a visually
hidden <label>, both get role="search" and both preserve the current
query in their input.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

resources/views/layouts/app.blade.php

@@ -1237,10 +1237,11 @@
 
     <!-- Mobile Search Overlay -->
     <div class="mobile-search-overlay" id="mobileSearchOverlay">
-        <form action="{{ route('videos.search') }}" method="GET" class="mobile-search-form">
-            <input type="text" name="q" class="mobile-search-input" placeholder="Search" value="{{ request('q') }}">
-            <button type="submit" class="mobile-search-submit">
-                <i class="bi bi-search"></i>
+        <form action="{{ route('search') }}" method="GET" class="mobile-search-form" role="search">
+            <label for="mobile-search-input" class="visually-hidden">Search</label>
+            <input type="search" id="mobile-search-input" name="q" class="mobile-search-input" placeholder="Search" value="{{ request('q') }}" aria-label="Search videos, channels and playlists">
+            <button type="submit" class="mobile-search-submit" aria-label="Submit search">
+                <i class="bi bi-search" aria-hidden="true"></i>
             </button>
         </form>
     </div>