feat: Stage 4 — on-call schedule calendar view

Adds a 14-day schedule list accessible via the Schedule tab.

Keybindings in schedule section:
  ←/→ (or h/l)  shift the 2-week window back/forward by one week
  j/k            navigate rows
  +              open user picker — select a user to assign to the date
  d              delete the selected day's assignment (y/N confirmation)
  r              refresh schedule from server

The user picker fetches the user list from the server on first open
(cached for the session). Selecting a user assigns them to the
highlighted date (POST /api/schedule all-or-nothing; 409 conflicts
surface as a status bar error).

The on-call header shows today's scheduled person and the current
window date range. On-call data refreshes on schedule actions.

API additions: GetSchedule, GetCurrentOnCall, AssignSchedule,
DeleteScheduleEntry, ListUsers. ScheduleEntry and User types added.

internal/api/types.go

@@ -48,3 +48,18 @@ type DayStat struct {
 	DayName string `json:"day_name"`
 	Count   int    `json:"count"`
 }
+
+type ScheduleEntry struct {
+	ID        int64     `json:"id"`
+	UserID    int64     `json:"user_id"`
+	Username  string    `json:"username"`
+	Date      string    `json:"date"` // YYYY-MM-DD
+	CreatedAt time.Time `json:"created_at"`
+}
+
+type User struct {
+	ID        int64     `json:"id"`
+	Username  string    `json:"username"`
+	Email     string    `json:"email"`
+	CreatedAt time.Time `json:"created_at"`
+}