refactor: extract shared cloud IPC message handler and fix Discord TS error (#143)

Two improvements in this PR:

1. **Extract `makeCloudMessageHandler` in `ipc.ts`** — the `processMessage`
   callback passed to both `startSpritesIpcPoller` and `startDaytonaIpcPoller`
   was copy-pasted verbatim (16 lines each), differing only in the log label.
   Extracted into a shared factory `makeCloudMessageHandler(deps)` that accepts
   a `backendLabel` string for the logger. Also extracts the `processTask`
   handler into a shared `cloudTaskHandler` constant.
   Net change: removes ~12 lines of duplication, single place to update routing
   logic when adding future cloud backends.

2. **Fix `mentionsOtherUsersOnly` type error in `discord.ts`** — `botId` is
   `string | undefined`, but `Map.has()` requires `string`. Added `botId != null`
   guard at the start of the expression so TypeScript can narrow the type.
   No behavior change (outer `if (!isDM && botId && ...)` already guarded it),
   but eliminates the pre-existing TS2345 error.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Peyton-Spencer