fix: use nullish coalescing to fix priority 0 bug display

Fixes display bug where P0 issues were shown as P2 due to JavaScript treating 0 as falsy. Also adds P0 to the priority filter dropdown.

Changes:
- Replace || with ?? operator for priority defaults in monitor UI
- Add P0 option to priority filter selector

Co-authored-by: Zack Rosen <zjrosen@users.noreply.github.com>

examples/monitor-webui/web/index.html

@@ -64,6 +64,7 @@
                 <div class="filter-group">
                     <label for="filter-priority">Priority</label>
                     <select id="filter-priority" multiple>
+                        <option value="0" selected>P0</option>
                         <option value="1" selected>P1</option>
                         <option value="2" selected>P2</option>
                         <option value="3" selected>P3</option>