Share platform-split module types via .shared.ts files

Extract the prop/control types (and the video error classes) that were
duplicated between web implementations and their native throw-stubs into
sibling .shared.ts files, so both variants reference one definition and
cannot drift.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BCuMKXWHuyGoNhTVAHBMyk

src/view/com/composer/videos/SubtitleFilePicker.native.tsx

@@ -1,6 +1,5 @@
-export function SubtitleFilePicker(_props: {
-  onSelectFile: (file: File) => void
-  disabled?: boolean
-}): never {
+import {type SubtitleFilePickerProps} from './SubtitleFilePicker.shared'
+
+export function SubtitleFilePicker(_props: SubtitleFilePickerProps): never {
   throw new Error('SubtitleFilePicker is a web-only component')
 }