fix(renderer): coerce strtotime() result to int before formatting dates

date() expects int|null, but strtotime() returns int|false; cast to make
the type explicit for PHPStan level 8 (the date strings are always
well-formed here, so strtotime() never actually returns false).

src/Service/Renderer/SvgRenderer.php

@@ -142,7 +142,7 @@ final class SvgRenderer
                 if ($cell === null) {
                     continue;
                 }
-                $ts    = strtotime($cell['date']);
+                $ts    = (int) strtotime($cell['date']);
                 $month = (int) date('n', $ts);
                 $dom   = (int) date('j', $ts);
 
@@ -197,7 +197,7 @@ final class SvgRenderer
                 $y     = self::MARGIN_Y + $d * self::STEP;
                 $color = $colors['levels'][$this->level($cell['count'])];
 
-                $ts     = strtotime($cell['date']);
+                $ts     = (int) strtotime($cell['date']);
                 $suffix = $cell['count'] !== 1 ? 's' : '';
                 $label  = $cell['count'] > 0
                     ? $cell['count'] . ' contribution' . $suffix . ' on ' . date('F j, Y', $ts)