Timers: "Group by source" on the All timers tile, off by default

Off, every timer sits in one grid filled row by row — a wrapping flex row
rather than a CSS grid, so the last row's cards widen to use the whole
width and nothing is left empty; a source in trouble is named in one line
above the cards, and its own cards dim. On, the sections per source come
back, and "Source headings" appears (showWhen) to hide their titles.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

modules/propresenter-timers/module.json

@@ -156,11 +156,18 @@
       "default": false,
       "group": "Show"
     },
+    "groupBySource": {
+      "type": "switch",
+      "label": "Group by source",
+      "default": false,
+      "group": "Show"
+    },
     "showHeadings": {
       "type": "switch",
       "label": "Source headings",
       "default": true,
-      "group": "Show"
+      "group": "Show",
+      "showWhen": "groupBySource"
     },
     "clock24h": {
       "type": "switch",