Add Usage Analytics view: tokens/questions/cost over time and by model

New large modal off the Stats sheet with a 6-way timeframe picker
(Today/7 Days/Week/Month/Year/Total), three tappable summary tiles,
and native Swift Charts (bar chart over time, pie chart by model).
Backed by date-range-filtered DatabaseService queries plus a new
daily-bucketed query — no schema changes needed since messages
already carry timestamp/tokens/cost/role.

oAI/Models/UsageStats.swift

@@ -25,6 +25,7 @@ import Foundation
 
 struct UsageStats: Sendable {
     var totalMessages: Int
+    var totalQuestions: Int
     var totalTokens: Int
     var totalCost: Double
     var hasCostData: Bool
@@ -33,6 +34,7 @@ struct UsageStats: Sendable {
 
     nonisolated init(
         totalMessages: Int = 0,
+        totalQuestions: Int = 0,
         totalTokens: Int = 0,
         totalCost: Double = 0.0,
         hasCostData: Bool = false,
@@ -40,6 +42,7 @@ struct UsageStats: Sendable {
         lastMessageDate: Date? = nil
     ) {
         self.totalMessages = totalMessages
+        self.totalQuestions = totalQuestions
         self.totalTokens = totalTokens
         self.totalCost = totalCost
         self.hasCostData = hasCostData
@@ -66,6 +69,7 @@ struct ModelUsageStat: Identifiable, Sendable {
     var id: String { modelId }
     let modelId: String
     var messageCount: Int
+    var questionCount: Int
     var totalTokens: Int
     var totalCost: Double
     var hasCostData: Bool
@@ -74,6 +78,7 @@ struct ModelUsageStat: Identifiable, Sendable {
     nonisolated init(
         modelId: String,
         messageCount: Int,
+        questionCount: Int = 0,
         totalTokens: Int,
         totalCost: Double,
         hasCostData: Bool,
@@ -81,6 +86,7 @@ struct ModelUsageStat: Identifiable, Sendable {
     ) {
         self.modelId = modelId
         self.messageCount = messageCount
+        self.questionCount = questionCount
         self.totalTokens = totalTokens
         self.totalCost = totalCost
         self.hasCostData = hasCostData
@@ -102,6 +108,33 @@ struct ModelUsageStat: Identifiable, Sendable {
     }
 }
 
+/// One day's usage totals, for time-series charting in the Analytics view.
+struct DailyUsageStat: Identifiable, Sendable {
+    var id: Date { day }
+    let day: Date
+    var messageCount: Int
+    var questionCount: Int
+    var totalTokens: Int
+    var totalCost: Double
+    var hasCostData: Bool
+
+    nonisolated init(
+        day: Date,
+        messageCount: Int,
+        questionCount: Int,
+        totalTokens: Int,
+        totalCost: Double,
+        hasCostData: Bool
+    ) {
+        self.day = day
+        self.messageCount = messageCount
+        self.questionCount = questionCount
+        self.totalTokens = totalTokens
+        self.totalCost = totalCost
+        self.hasCostData = hasCostData
+    }
+}
+
 struct ConversationUsageStat: Identifiable, Sendable {
     let conversationId: UUID
     var id: UUID { conversationId }
@@ -141,3 +174,33 @@ struct ConversationUsageStat: Identifiable, Sendable {
         hasCostData ? String(format: "$%.4f", totalCost) : "N/A"
     }
 }
+
+/// Time window for the Analytics view. `.total` means all-time (no lower bound).
+enum AnalyticsTimeframe: String, CaseIterable, Identifiable, Sendable {
+    case today
+    case last7Days
+    case week
+    case month
+    case year
+    case total
+
+    var id: String { rawValue }
+
+    /// Bounds for this timeframe. `start` is nil for `.total` (all time, no lower bound).
+    nonisolated func dateRange(now: Date = Date(), calendar: Calendar = .current) -> (start: Date?, end: Date) {
+        switch self {
+        case .today:
+            return (calendar.startOfDay(for: now), now)
+        case .last7Days:
+            return (calendar.date(byAdding: .day, value: -7, to: now) ?? now, now)
+        case .week:
+            return (calendar.dateInterval(of: .weekOfYear, for: now)?.start ?? now, now)
+        case .month:
+            return (calendar.dateInterval(of: .month, for: now)?.start ?? now, now)
+        case .year:
+            return (calendar.dateInterval(of: .year, for: now)?.start ?? now, now)
+        case .total:
+            return (nil, now)
+        }
+    }
+}