fix(desktop): show skeleton during channel switch

[tlongwell-block]

Summary

• carry the channel id alongside the deferred message snapshot in MessageTimeline
• treat deferred snapshots from a previous channel as loading so the main pane paints the skeleton during route/channel switches
• add tests for stale deferred channel snapshots and skeleton selection

Test plan

• bin/node --import ./desktop/test-loader.mjs --experimental-strip-types --test desktop/src/features/messages/lib/timelineSnapshot.test.mjs desktop/src/features/messages/lib/timelineLoadingState.test.mjs
• bin/pnpm --dir desktop typecheck
• pre-commit hooks: cargo fmt --all, cargo fmt --manifest-path desktop/src-tauri/Cargo.toml --all, web/desktop biome + file-size checks, mobile format/analyze
• pre-push hooks: desktop/mobile/rust/desktop-tauri tests