refactor: consolidate IPC types into src/shared/types.ts

The 12 cross-process data types were manually copied across
src/preload/index.ts and src/renderer/src/types/t4sd.d.ts. A silent
field addition or rename in either file would not produce a compile
error in the other — the divergence only surfaces at runtime.

A shared module makes the compiler enforce consistency. Preload imports
from shared/types and re-exports for downstream consumers. The renderer's
t4sd.d.ts re-exports the same types from shared/types, so all existing
screen imports continue to work without modification.

Both tsconfigs are extended to include src/shared so the module is
reachable from both composite projects.

src/main/playback/types.ts

@@ -8,10 +8,4 @@ export type Quality =
   | 'audio_only'
   | 'worst'
 
-export interface PlaybackEvent {
-  kind: 'started' | 'stopped' | 'error'
-  channelLogin?: string
-  message?: string
-  durationSeconds?: number
-  isLive?: boolean
-}
+export type { PlaybackEvent } from '../../shared/types'