Refactor: unify capture filename assembly into CaptureFilename

Sources/Stag/Capture/CaptureFilename.swift

@@ -0,0 +1,14 @@
+import Foundation
+
+/// Assembles capture filenames from their parts so the real save path
+/// (`CaptureManager`) and the settings preview (`PreferencesWindow`) share one
+/// rule and can't drift. Format: `<prefix><slug + space?><timestamp>.<ext>`.
+enum CaptureFilename {
+    /// Builds a capture filename. An empty `prefix` falls back to `"Stag_"`; an
+    /// empty `slug` is omitted along with its separating space.
+    static func make(prefix: String, slug: String, timestamp: String, ext: String) -> String {
+        let resolvedPrefix = prefix.isEmpty ? "Stag_" : prefix
+        let middle = slug.isEmpty ? "" : "\(slug) "
+        return "\(resolvedPrefix)\(middle)\(timestamp).\(ext)"
+    }
+}

Sources/Stag/CaptureManager.swift

@@ -363,13 +363,16 @@ final class CaptureManager {
         let prefs = store.preferences
         let saveDir = URL(fileURLWithPath: prefs.expandedSavePath)
         try? FileManager.default.createDirectory(at: saveDir, withIntermediateDirectories: true)
-        let ext = prefs.defaultFormat == .jpeg ? "jpg" : "png"
-        let prefix = prefs.filePrefix.isEmpty ? "Stag_" : prefs.filePrefix
         // Smart filename: insert the source app (e.g. "Slack") between the prefix
         // and the timestamp when available — falls back to prefix+timestamp.
         let slug = prefs.useSmartFilenames ? CaptureContext.shared.filenameSlug() : ""
-        let middle = slug.isEmpty ? "" : "\(slug) "
-        let url = saveDir.appendingPathComponent("\(prefix)\(middle)\(Date().shotTimestamp).\(ext)")
+        let filename = CaptureFilename.make(
+            prefix: prefs.filePrefix,
+            slug: slug,
+            timestamp: Date().shotTimestamp,
+            ext: prefs.defaultFormat.fileExtension
+        )
+        let url = saveDir.appendingPathComponent(filename)
 
         switch prefs.defaultFormat {
         case .png: image.pngWrite(to: url)

Sources/Stag/Models/Preferences.swift

@@ -5,6 +5,14 @@ import Combine
 
 enum CaptureFormat: String, Codable, CaseIterable {
     case png, jpeg
+
+    /// File extension for saved captures ("png" / "jpg").
+    var fileExtension: String {
+        switch self {
+        case .png:  return "png"
+        case .jpeg: return "jpg"
+        }
+    }
 }
 
 enum AfterCaptureAction: String, Codable, CaseIterable {

Sources/Stag/Views/PreferencesWindow.swift

@@ -599,9 +599,13 @@ private struct PreferencesView: View {
     // MARK: Helpers
 
     private var filenamePreview: String {
-        let prefix = prefs.filePrefix.isEmpty ? "Stag_" : prefs.filePrefix
-        let mid = prefs.useSmartFilenames ? "Safari " : ""
-        return "\(prefix)\(mid)2026-01-01.png"
+        let slug = prefs.useSmartFilenames ? "Safari" : ""
+        return CaptureFilename.make(
+            prefix: prefs.filePrefix,
+            slug: slug,
+            timestamp: "2026-01-01",
+            ext: prefs.defaultFormat.fileExtension
+        )
     }
 
     private func actionDisplayName(_ action: AfterCaptureAction) -> String {

Tests/StagTests/CaptureFilenameTests.swift

@@ -0,0 +1,46 @@
+import XCTest
+@testable import Stag
+
+/// Filename assembly shared by CaptureManager (real save) and PreferencesWindow
+/// (settings preview), plus CaptureFormat's extension mapping.
+final class CaptureFilenameTests: XCTestCase {
+
+    func testEmptyPrefixFallsBackToStag() {
+        XCTAssertEqual(
+            CaptureFilename.make(prefix: "", slug: "", timestamp: "2026-01-01", ext: "png"),
+            "Stag_2026-01-01.png"
+        )
+    }
+
+    func testCustomPrefixIsUsedVerbatim() {
+        XCTAssertEqual(
+            CaptureFilename.make(prefix: "Shot-", slug: "", timestamp: "2026-01-01", ext: "png"),
+            "Shot-2026-01-01.png"
+        )
+    }
+
+    func testSlugInsertedWithSeparatingSpace() {
+        XCTAssertEqual(
+            CaptureFilename.make(prefix: "", slug: "Slack", timestamp: "T", ext: "png"),
+            "Stag_Slack T.png"
+        )
+    }
+
+    func testEmptySlugOmitsSpace() {
+        let result = CaptureFilename.make(prefix: "Stag_", slug: "", timestamp: "T", ext: "jpg")
+        XCTAssertEqual(result, "Stag_T.jpg")
+        XCTAssertFalse(result.contains("  "))
+    }
+
+    func testExtensionIsApplied() {
+        XCTAssertEqual(
+            CaptureFilename.make(prefix: "P", slug: "App", timestamp: "ts", ext: "jpg"),
+            "PApp ts.jpg"
+        )
+    }
+
+    func testCaptureFormatFileExtension() {
+        XCTAssertEqual(CaptureFormat.png.fileExtension, "png")
+        XCTAssertEqual(CaptureFormat.jpeg.fileExtension, "jpg")
+    }
+}