Redesign Calendar + Messages, fix favicon, add date picker, Tasks & Activity

- Appointments now carry a date; the Calendar button opens a Google-Calendar
  style month grid (color-coded event chips, month nav, Today, click a day for
  its list). Today/Upcoming sections derive from dates.
- Add-appointment dialog gains a date picker (Popover + Calendar).
- Messages rebuilt as chat threads: an input-based composer that appends to a
  two-way timeline (no more toast covering the field), and the unread count is
  now a filter toggle.
- Favicon: the brand logo now shows in the tab. Removed the stale default
  app/favicon.ico (which shadowed it) and added app/icon.png + app/apple-icon.png
  generated from the logo; dropped the redundant metadata.icons.
- Sidebar: swapped Notes <-> Messages order (both kept).
- New pages: Activity (signed-change audit timeline with hash chips + approval
  status) and Tasks (two-pane care-team to-do with add dialog).
- Prescriptions: adding a medication now surfaces mock drug-interaction and
  allergy warnings against the patient's record.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/i18n/locales/en/translation.json

@@ -52,6 +52,8 @@
     "analysis": "Analysis",
     "notes": "Notes",
     "messages": "Messages",
+    "tasks": "Tasks",
+    "activity": "Activity",
     "settings": "Settings",
     "notifications": "Notifications",
     "viewAllNotifications": "View all notifications",

frontend/lib/nav.ts

@@ -1,6 +1,8 @@
 import {
   BarChart3,
   CalendarClock,
+  History,
+  ListTodo,
   type LucideIcon,
   Mail,
   NotebookPen,
@@ -60,7 +62,9 @@ export const navItems: NavItem[] = [
     icon: BarChart3,
     link: "/analysis",
   },
-  { id: "notes", labelKey: "nav.notes", icon: NotebookPen, link: "/notes" },
   { id: "messages", labelKey: "nav.messages", icon: Mail, link: "/messages" },
+  { id: "notes", labelKey: "nav.notes", icon: NotebookPen, link: "/notes" },
+  { id: "tasks", labelKey: "nav.tasks", icon: ListTodo, link: "/tasks" },
+  { id: "activity", labelKey: "nav.activity", icon: History, link: "/activity" },
   { id: "settings", labelKey: "nav.settings", icon: Settings, link: "/settings" },
 ];