fix: history load notifies and never overwrites unreadable file; dictionary keeps ids across external edits, undo safety

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Tests/SstDataTests/ViewSupportTests.swift

@@ -67,6 +67,20 @@ struct HistoryViewModelTests {
         return store
     }
 
+    @Test func daysAreFilledAfterTheStoreLoads() {
+        let url = FileManager.default.temporaryDirectory.appending(path: "sst-vm-\(UUID().uuidString)/h.json")
+        let writer = HistoryStore(fileURL: url)
+        writer.add(HistoryEntry(date: .now, duration: 1, rawText: "x", finalText: "x"))
+        writer.flush()
+        let store = HistoryStore(fileURL: url)
+        let model = HistoryViewModel(store: store)
+        store.onChange = { model.refresh() }
+        #expect(model.days.isEmpty)
+        store.load()
+        #expect(!model.days.isEmpty)
+        #expect(model.totalCount == 1)
+    }
+
     @Test func listsAllEntriesNewestFirstAndFiltersByQuery() {
         let model = HistoryViewModel(store: store(["eerste", "tweede zin", "derde"]))
         #expect(model.totalCount == 3)
@@ -152,6 +166,31 @@ struct DictionaryViewModelTests {
         #expect(store.entries == [term("Supabase")])
     }
 
+    @Test func externalChangeClearsUndo() throws {
+        let (model, store) = try makeModel("Supabase\n")
+        model.add(.term("Vercel"))
+        #expect(model.canUndo)
+        model.externalChangeDetected()
+        #expect(!model.canUndo)
+        model.undo()
+        #expect(store.entries == [term("Supabase"), term("Vercel")])
+    }
+
+    @Test func failedUndoKeepsTheSnapshot() throws {
+        let (model, store) = try makeModel("Supabase\n")
+        model.add(.term("Vercel"))
+        let folder = store.fileURL.deletingLastPathComponent()
+        try FileManager.default.setAttributes([.posixPermissions: 0o500], ofItemAtPath: folder.path)
+        model.undo()
+        try FileManager.default.setAttributes([.posixPermissions: 0o700], ofItemAtPath: folder.path)
+        #expect(model.errorMessage != nil)
+        #expect(model.canUndo)
+        model.errorMessage = nil
+        model.undo()
+        #expect(store.entries == [term("Supabase")])
+        #expect(!model.canUndo)
+    }
+
     @Test func searchMatchesBothSidesIgnoringCaseAndDiacritics() throws {
         let (model, _) = try makeModel("cloud code => Claude Code\nCafé\nVercel\n")
         model.query = "CLAUDE"