feat: app bar and thread drawer in the example

[divyanshub024]

Summary

The example gets app chrome: a flat AppBar in the Flow tokens with the thread's title (its opening question), a drawer button and a new-chat action, and a Drawer holding a FlowThreadList over the screen's conversations. Stacked on #37.

• The screen now holds a map of threads; _messages is the active one, so the chat wiring is unchanged. Switching threads stops any stream and clears the pending attachments.
• One package change alongside: FlowChatView re-reads its jump-to-latest state from the thread's first metrics, so a thread keyed per conversation does not carry the last one's button.

Screenshots

App bar	Drawer
attached separately	attached separately

How this was verified

• iPhone 17 Pro simulator: title updates on send, the drawer lists the thread under Today with the selection fill, new chat from the bar and the drawer.
• flutter analyze clean in example/; dart format . applied.

Checklist

• flutter analyze lib and flutter analyze in example/ and playground/ are clean
• dart format . applied
• Exercised in the playground — with a stage demo added or updated if this is a new component or variant
• Any new entry under dependencies: in pubspec.yaml is flutter.dev-published, forces no configuration on hosts that never use the feature, and is argued in this PR
• Nothing model-facing — no prompts, schemas, or provider/network calls
• New public API is exported from lib/flow_ui.dart and documented in docs/ and the README table
• CHANGELOG.md updated for user-facing changes, with breaking changes called out
• PR title follows conventional commits (feat:, fix:, refactor:, docs:, chore:)

---

Note

Low Risk
Example-app-only UI and local state; no package API or auth/network changes.

Overview
The example chat screen gains multi-conversation hosting and navigation chrome, without changing the flow_ui package.

Message state moves from a single list into a thread map with an active id; _messages stays a getter/setter on the active thread so send/stream logic is largely unchanged. Users can open threads and start new chats from a Flow-styled AppBar (title from the first user message) and a drawer wired to FlowThreadList. Per-thread composer drafts are saved when switching; switching also stops in-flight generation and clears pending attachments. Streaming and late image measurement call _update with the originating thread id so replies still land on the right conversation if the user navigates away mid-stream. FlowThread gets a ValueKey per thread for separate scroll state, and _stop no-ops when nothing is generating.

^{Reviewed by Cursor Bugbot for commit 1809303. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 1809303. Configure here.}