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/assets/theme.css

@@ -4,6 +4,7 @@
   --color-bg: #f5f5fb;
   --color-bg-secondary: #ededf5;
   --color-bg-card: #ffffff;
+  --color-surface: #f0f0f8;
   --color-text: #1a1a1a;
   --color-text-secondary: #666666;
   --color-text-muted: #999999;
@@ -55,6 +56,7 @@
   --color-bg: #111113;
   --color-bg-secondary: #18181f;
   --color-bg-card: #1e1e27;
+  --color-surface: #1a1b22;
   --color-text: #e4e4f0;
   --color-text-secondary: #8888a8;
   --color-text-muted: #52526a;