fix(core): transport-aware launch paths + writable-folder check (adversarial review)

BLOCKER fix, Core half: adds TransportSettings.resolvedAppSupportPaths(),
the transport-aware equivalent of the AirDrop-only
FolderSettings.resolvedAppSupportPaths() — launch code must use this one so
the ReturnWatcher it feeds is never seeded with a stale AirDrop folder while
OneDrive is the persisted transport. Both now share a single
AppSupportPaths.standardRoot() helper for the ~/Library/Application
Support/Shotdeck root, instead of computing it three separate times.

MAJOR fix, Core half: adds OneDriveLocator.isWritableDirectory(at:) — exists
+ isDirectory is not enough; an existing-but-unwritable folder (permissions
revoked) must be treated as unavailable, not silently attempted and surfaced
as a generic PDF-composition failure.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ZiTXPbPCSjzPVsfoweAbp

Sources/ShotdeckCore/Support/AppSupportPaths.swift

@@ -11,12 +11,6 @@ public struct AppSupportPaths: Sendable {
     /// Production paths.
     public static func standard() throws -> AppSupportPaths {
         let fileManager = FileManager.default
-        let appSupportParent = try fileManager.url(
-            for: .applicationSupportDirectory,
-            in: .userDomainMask,
-            appropriateFor: nil,
-            create: true
-        )
         let desktop = try fileManager.url(
             for: .desktopDirectory,
             in: .userDomainMask,
@@ -29,10 +23,25 @@ public struct AppSupportPaths: Sendable {
             appropriateFor: nil,
             create: true
         )
-        let root = appSupportParent.appendingPathComponent("Shotdeck", isDirectory: true)
+        let root = try standardRoot(fileManager: fileManager)
         return try AppSupportPaths(root: root, outbox: desktop, watchFolder: downloads)
     }
 
+    /// The standard `~/Library/Application Support/Shotdeck` root. Shared by
+    /// `standard()`, `FolderSettings.resolvedAppSupportPaths()`, and
+    /// `TransportSettings.resolvedAppSupportPaths()` so all three agree on where the
+    /// root lives — the folder-resolution logic (AirDrop-only vs transport-aware)
+    /// differs between those, the root computation never should.
+    public static func standardRoot(fileManager: FileManager = .default) throws -> URL {
+        let appSupportParent = try fileManager.url(
+            for: .applicationSupportDirectory,
+            in: .userDomainMask,
+            appropriateFor: nil,
+            create: true
+        )
+        return appSupportParent.appendingPathComponent("Shotdeck", isDirectory: true)
+    }
+
     /// Test paths rooted anywhere. Every directory is created if missing.
     public init(root: URL, outbox: URL, watchFolder: URL) throws {
         self.root = root