Fix the security audit's findings

ShareDownload::$authenticated was a public property without #[Locked],
so a visitor with a share's link but not its password could set it from
the browser and see the file list: names, folder paths, sizes and
whether the share holds a text. The contents stayed protected; the flag
is now locked, and a test tries to set it.

Sessions hold the keys of the password shares a recipient unlocked, and
were stored unencrypted in the sessions table wherever SESSION_ENCRYPT
was not set, which includes every Docker install. They are now
encrypted unless SESSION_ENCRYPT=false. Share keys never live in the
session, so every share stays readable; people sign in again once.

The container generated an APP_KEY when none was set, printed it to the
logs and generated another on the next start, which left the shares
without a password unreadable. It now refuses to start without one and
says how to create it; docker-compose.example.yml requires it for both
services. The command it names, and the README's, now pass
--entrypoint php: the image's entrypoint ignores its arguments, so
"docker run <image> php artisan key:generate --show" never ran it.

Every proxy is trusted, so the password-reset mail built its link from
a host a forged X-Forwarded-Host could choose. It is now built from
APP_URL.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/Livewire/ShareDownload.php

@@ -7,6 +7,7 @@ use App\Services\ShareService;
 use Carbon\CarbonInterval;
 use Illuminate\Support\Facades\RateLimiter;
 use Livewire\Attributes\Layout;
+use Livewire\Attributes\Locked;
 use Livewire\Attributes\Renderless;
 use Livewire\Attributes\Validate;
 use Livewire\Component;
@@ -16,6 +17,8 @@ class ShareDownload extends Component
 {
     public Share $share;
 
+    /** Set only by mount() and verifyPassword(): the browser must never unlock the file list. */
+    #[Locked]
     public bool $authenticated = false;
 
     #[Validate('required|string')]