untested... test: add unit test suite and CI workflow

I have never done this style of test before... who knows if it will work

- Add Gitea Actions workflow for PHP 8.2 testing
- Add PHPUnit configuration and bootstrap file
- Add unit tests for TalkService, WebhookController, ImageCleanup, NavigationProvider, and DebugToggle
- Add README documentation for the test suite
- Update composer.json with test and lint scripts

phpunit.xml.dist

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="utf-8"?>
+<phpunit
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.0/phpunit.xsd"
+    bootstrap="tests/bootstrap.php"
+    cacheDirectory=".phpunit.cache"
+    beStrictAboutOutputDuringTests="true"
+    failOnRisky="true"
+    failOnWarning="true"
+>
+    <testsuites>
+        <testsuite name="unit">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+
+    <source>
+        <include>
+            <directory suffix=".php">lib</directory>
+        </include>
+    </source>
+</phpunit>