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]>

routes/web.php

@@ -188,6 +188,8 @@ Route::middleware(['auth', 'verified'])->group(function () {
 
 // Channel - public for viewing. {username} is the user's slug (non-sequential, safe to share).
 Route::get('/channel/{username?}', [UserController::class, 'channel'])->name('channel');
+Route::get('/channel/{username}/videos/more', [UserController::class, 'channelVideosMore'])
+    ->name('channel.videos.more');
 // YouTube-style @handle alias. For now `handle` is just the existing
 // `username` slug — if the two ever diverge, flip the controller lookup
 // in UserController::channel() to prefer a dedicated `handle` column.
@@ -197,6 +199,20 @@ Route::get('/@{handle}', [UserController::class, 'channel'])
     ->where('handle', '[A-Za-z0-9][A-Za-z0-9._-]*')
     ->name('channel.handle');
 
+// Tab aliases: /@handle/videos lands on the Videos tab, /@handle/shorts on
+// Shorts, etc. These bridge to the single-page channel view by forwarding
+// the tab name as a hash — the channel.blade.php JS already handles
+// #wall/#videos/#shorts/#playlists/#about/#posts on load.
+Route::get('/@{handle}/{tab}', function (string $handle, string $tab) {
+    $allowed = ['wall', 'videos', 'shorts', 'playlists', 'posts', 'about'];
+    if (! in_array($tab, $allowed, true)) {
+        abort(404);
+    }
+    $user = \App\Models\User::where('username', $handle)->firstOrFail();
+    return redirect(route('channel', $user->channel) . '#' . $tab, 302);
+})->where(['handle' => '[A-Za-z0-9][A-Za-z0-9._-]*', 'tab' => '[a-z]+'])
+  ->name('channel.handle.tab');
+
 // Post routes - auth required
 Route::middleware(['auth'])->group(function () {
     Route::post('/channel/{user}/posts', [PostController::class, 'store'])->name('posts.store');