Show everything past next week on Calendar, grouped by month

The agenda used to hard-cut at a ~14-day lookahead with no way to see
anything past it. Drop the cutoff and bucket whatever falls beyond
"Next week" into per-month groups (each its own sticky header), so the
screen stays a full upcoming agenda instead of losing events that were
still worth seeing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

static/css/app.css

@@ -4403,6 +4403,9 @@
   .self-start {
     align-self: flex-start;
   }
+  .self-stretch {
+    align-self: stretch;
+  }
   .truncate {
     overflow: hidden;
     text-overflow: ellipsis;