fix(capabilities): cover transcription-preview window in default ACL

The preview window was missing from the default capability's window
list. Its frontend calls core:window:allow-hide (getCurrentWindow().hide()
on dismiss / auto-hide) plus invoke('copy_to_clipboard') and a handful
of event listeners — none of those would have been permitted at runtime
because the capability never matched this window's label.

Add 'transcription-preview' alongside the other secondary windows so
the preview actually has access to the permissions it already relies on.
Regenerate gen/schemas/capabilities.json to match.

Co-authored-by: jars <jakejars@users.noreply.github.com>

src-tauri/capabilities/default.json

@@ -2,7 +2,7 @@
   "$schema": "../gen/schemas/desktop-schema.json",
   "identifier": "default",
   "description": "Capability for the main window",
-  "windows": ["main", "tasks-float", "transcript-viewer"],
+  "windows": ["main", "tasks-float", "transcript-viewer", "transcription-preview"],
   "permissions": [
     "core:default",
     "core:window:allow-start-dragging",