refactor: DRY calendar additions + define --color-surface in theme

- Define --color-surface in theme.css (light: #f0f0f8, dark: #1a1b22)
  — was used across 10+ components with no-op fallbacks; now properly
  defined alongside --color-bg and --color-bg-card
- Extract shared date formatters into utils/dateFormat.ts:
  fmtTime, fmtDateTime, fmtRelativeDateTime, fmtDayLabel, fmtCompact
- Replace duplicate inline formatters in CalendarView, HomeView
  (formatUpcomingTime), and KnowledgeView (formatEventDate)
- CalendarView: replace hardcoded rgba(99,102,241,0.4) hover colour
  with color-mix(in srgb, var(--color-primary) 40%, transparent);
  fix --color-input-bg fallback to use var(--color-bg); remove
  hardcoded hex fallbacks now that --color-surface is defined

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/HomeView.vue

@@ -3,6 +3,7 @@ import { ref, computed, onMounted, onUnmounted } from "vue";
 import { apiGet, listEvents } from "@/api/client";
 import { useBackgroundRefresh } from "@/composables/useBackgroundRefresh";
 import { milestoneColor } from "@/utils/palette";
+import { fmtRelativeDateTime } from "@/utils/dateFormat";
 import type { Note } from "@/types/note";
 import type { Task, TaskListResponse, TaskStatus } from "@/types/task";
 import type { ToolCallRecord, Message } from "@/types/chat";
@@ -304,28 +305,8 @@ function onEventDeleted(id: number) {
 }
 
 function formatUpcomingTime(event: EventEntry): string {
-  if (event.all_day) return "All day";
   if (!event.start_dt) return "";
-  try {
-    const d = new Date(event.start_dt);
-    const today = new Date();
-    const tomorrow = new Date(today);
-    tomorrow.setDate(today.getDate() + 1);
-    const isToday =
-      d.getFullYear() === today.getFullYear() &&
-      d.getMonth() === today.getMonth() &&
-      d.getDate() === today.getDate();
-    const isTomorrow =
-      d.getFullYear() === tomorrow.getFullYear() &&
-      d.getMonth() === tomorrow.getMonth() &&
-      d.getDate() === tomorrow.getDate();
-    const timeStr = d.toLocaleTimeString(undefined, { hour: "numeric", minute: "2-digit" });
-    if (isToday) return `Today ${timeStr}`;
-    if (isTomorrow) return `Tomorrow ${timeStr}`;
-    return d.toLocaleDateString(undefined, { weekday: "short", month: "short", day: "numeric" }) + " " + timeStr;
-  } catch {
-    return event.start_dt;
-  }
+  return fmtRelativeDateTime(event.start_dt, event.all_day);
 }
 </script>