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/Localizable.xcstrings

@@ -1,6 +1,40 @@
 {
   "sourceLanguage" : "en",
   "strings" : {
+    "By Provider" : {
+      "localizations" : {
+        "da" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Efter udbyder"
+          }
+        },
+        "de" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Nach Anbieter"
+          }
+        },
+        "fr" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Par fournisseur"
+          }
+        },
+        "nb" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Etter leverandør"
+          }
+        },
+        "sv" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Efter leverantör"
+          }
+        }
+      }
+    },
     "Today" : {
       "localizations" : {
         "da" : {