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/FolderSettings.swift

@@ -70,15 +70,7 @@ public enum FolderSettings {
         defaults: UserDefaults = .standard,
         fileManager: FileManager = .default
     ) throws -> AppSupportPaths {
-        let resolvedRoot: URL
-        if let root {
-            resolvedRoot = root
-        } else {
-            let appSupportParent = try fileManager.url(
-                for: .applicationSupportDirectory, in: .userDomainMask,
-                appropriateFor: nil, create: true)
-            resolvedRoot = appSupportParent.appendingPathComponent("Shotdeck", isDirectory: true)
-        }
+        let resolvedRoot = try root ?? AppSupportPaths.standardRoot(fileManager: fileManager)
         let folders = resolve(defaults: defaults, fileManager: fileManager)
         return try AppSupportPaths(root: resolvedRoot, outbox: folders.outbox, watchFolder: folders.watch)
     }