Track usage independent of conversation save state; fix Analytics toolbar button

Conversation text is still only persisted when explicitly saved (⌘S),
but tokens/cost/model/provider are now logged to a new usage_events
table for every completed AI response regardless — the Analytics
view now reads from this table instead of messages, so it reflects
real usage even for conversations that were never saved. Adds a
By Provider chart mode alongside Over Time/By Model.

Also fixes the Analytics entry point: ToolbarItem(placement: .navigation)
silently doesn't render in a plain .sheet-presented NavigationStack on
macOS. Moved the button inline next to the segmented picker, matching
the codebase's existing convention (e.g. the model-favorites star filter).

oAI/Models/UsageStats.swift

@@ -108,6 +108,47 @@ struct ModelUsageStat: Identifiable, Sendable {
     }
 }
 
+/// Usage grouped by provider (openrouter/anthropic/openai/ollama/appleOnDevice), from `usage_events`.
+struct ProviderUsageStat: Identifiable, Sendable {
+    var id: String { provider }
+    let provider: String
+    var questionCount: Int
+    var totalTokens: Int
+    var totalCost: Double
+    var hasCostData: Bool
+    var lastUsed: Date
+
+    nonisolated init(
+        provider: String,
+        questionCount: Int,
+        totalTokens: Int,
+        totalCost: Double,
+        hasCostData: Bool,
+        lastUsed: Date
+    ) {
+        self.provider = provider
+        self.questionCount = questionCount
+        self.totalTokens = totalTokens
+        self.totalCost = totalCost
+        self.hasCostData = hasCostData
+        self.lastUsed = lastUsed
+    }
+
+    var totalTokensDisplay: String {
+        if totalTokens >= 1_000_000 {
+            return String(format: "%.1fM", Double(totalTokens) / 1_000_000)
+        } else if totalTokens >= 1000 {
+            return String(format: "%.1fK", Double(totalTokens) / 1000)
+        } else {
+            return "\(totalTokens)"
+        }
+    }
+
+    var totalCostDisplay: String {
+        hasCostData ? String(format: "$%.4f", totalCost) : "N/A"
+    }
+}
+
 /// One day's usage totals, for time-series charting in the Analytics view.
 struct DailyUsageStat: Identifiable, Sendable {
     var id: Date { day }