Feat/chat viewer bridge and bubble widget

[Nixxx19]

what this fixes

two connected problems, solved together:

1. silent data loss in dispatcher.rs
MofaNodeType::ChatViewer was hitting continue — every chat
message from dora was silently dropped. no error, no log, nothing.
implements ChatViewerBridge to complete the pipeline end-to-end.

2. chat bubble widget
adds ChatBubble (role-colored gpu-rendered bubble) and
ChatBubblePanel (scrollable conversation view) with full
user / assistant / system role support.

why one pr

the bridge without the widget = messages go nowhere.
the widget without the bridge = nothing feeds it.
one feature, split across two crates.

changes

• mofa-dora-bridge/src/widgets/chat_viewer_bridge.rs (new)
• mofa-dora-bridge/src/dispatcher.rs — replaces todo+continue
• mofa-ui/src/widgets/chat_bubble.rs (new)
• apps/mofa-test-app/src/main.rs — registry count 13→15
• fix pre-existing warnings in log_panel and sidebar

verified

• 18 tests pass, 0 regressions
• mofa-test-app: 14/14 green, all 15 widgets confirmed

[Nixxx19]

cc @BH3GEI @ymote please review, thanks!