feat(channel): /@handle/{tab} aliases + cursor pagination on Videos tab

Two complementary additions that share the same bridge-not-rewrite
philosophy — reuse the existing single-page channel view instead of
splitting its 2000 lines into five route-driven partials.

/@handle/{tab}: videos / shorts / playlists / posts / about / wall
redirect to the equivalent /channel/{slug}#tab URL (302). The channel
page's existing hash-tab JS already handles landing on the right tab;
these aliases just make the URLs shareable in the YouTube style.

Channel Videos tab Load More: new JSON endpoint
GET /channel/{username}/videos/more returns the next cursor page of
server-rendered video cards. A sentinel + IntersectionObserver below
the grid fetches the next page when scrolled into view, with a Load
More button as fallback. Behavior mirrors the Phase 2 home feed —
users with thousands of uploads no longer get a 300-item grid on
first paint.

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

resources/views/user/channel.blade.php

@@ -701,6 +701,56 @@ $headerSocialMap = [
             <span>No videos match your search</span>
         </div>
     </div>
+
+    {{-- Cursor-paginated Load More. The sentinel sits below the grid and
+         fetches the next slice when scrolled into view. A manual Load More
+         button is the fallback for browsers without IntersectionObserver. --}}
+    <div id="chVidSentinel"
+         data-username="{{ $user->username }}"
+         data-more-url="{{ route('channel.videos.more', ['username' => $user->username]) }}"
+         style="height:1px;"></div>
+    <div id="chVidLoadMoreWrap" style="display:none; text-align:center; padding:16px 0;">
+        <button type="button" id="chVidLoadMoreBtn" class="action-btn">Load more</button>
+    </div>
+    <script>
+    (function () {
+        const grid     = document.getElementById('chVidGrid');
+        const sentinel = document.getElementById('chVidSentinel');
+        const btnWrap  = document.getElementById('chVidLoadMoreWrap');
+        const btn      = document.getElementById('chVidLoadMoreBtn');
+        if (!grid || !sentinel) return;
+        let cursor = '', busy = false, done = false;
+        const url = sentinel.dataset.moreUrl;
+        async function loadMore() {
+            if (busy || done) return;
+            busy = true;
+            try {
+                const u = new URL(url, window.location.origin);
+                if (cursor) u.searchParams.set('cursor', cursor);
+                const r = await fetch(u.toString(), {
+                    headers: { 'X-Requested-With': 'XMLHttpRequest', 'Accept': 'application/json' },
+                    credentials: 'same-origin',
+                });
+                if (!r.ok) throw new Error('bad');
+                const d = await r.json();
+                if (d.items) grid.insertAdjacentHTML('beforeend', d.items);
+                cursor = d.next_cursor || '';
+                done   = !d.has_more;
+                if (done) { sentinel.style.display = 'none'; btnWrap.style.display = 'none'; }
+            } catch (e) {
+                btnWrap.style.display = 'block';
+            } finally { busy = false; }
+        }
+        btn?.addEventListener('click', loadMore);
+        if ('IntersectionObserver' in window) {
+            new IntersectionObserver(entries => {
+                if (entries.some(e => e.isIntersecting)) loadMore();
+            }, { rootMargin: '400px 0px' }).observe(sentinel);
+        } else {
+            btnWrap.style.display = 'block';
+        }
+    })();
+    </script>
     @else
     <div class="ch-empty">
         <div class="ch-empty-icon"><i class="bi bi-camera-video"></i></div>