Move the navigation into a floating toolbar at the bottom

The top app bar repeated the site's name, which already heads the upload
and download pages, and spread its actions to the window's far edge. A
floating toolbar centred above the bottom edge, as wide as its buttons,
now holds them: Upload and the admin pages with the current one filled,
and the account menu; guests get Upload, the theme toggle and Log in,
without tooltips. On a phone the sign-in card no longer stretches to the
full height, and the admin dashboard's empty state sits outside the
scrolling table.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V9NnLxnPp8vaaurb3Z1MFy

tests/Feature/Admin/AdminDashboardTest.php

@@ -87,3 +87,12 @@ test('the shares table sorts only by its own columns', function () {
 
     expect(Share::query()->count())->toBe(2);
 });
+
+test('without shares the dashboard shows an empty state instead of the table', function () {
+    $admin = User::query()->where('is_admin', true)->first();
+
+    $this->actingAs($admin)->get(route('admin.dashboard'))
+        ->assertOk()
+        ->assertSee('No shares yet')
+        ->assertDontSee('<table', false);
+});