feat: add retry button to TUI error messages

[dgageot]

What

Adds a clickable ↻ retry button to error messages in the TUI. When an agent turn fails (fatal model error, hook block, loop detection, tool-setup failure), the user can click the button to resume the conversation from where it left off — without retyping the last message.

Why

Previously, when the TUI showed an error in the messages view, there was no way to continue the conversation after a failure. The user had to re-enter their message manually. This adds a one-click recovery path.

How

• pkg/tui/types/types.go: new ErrorRetryLabel ("↻ retry") constant.
• pkg/tui/components/message/message.go: error messages render the retry affordance on a trailing line.
• pkg/tui/messages/edit.go: new RetryMsg event.
• pkg/tui/components/messages/messages.go: isRetryLabelClick detects clicks on the label and emits RetryMsg.
• pkg/app/app.go: new App.Retry re-runs RunStream on the existing session (no new user message), so the conversation continues from where it stopped.
• pkg/tui/page/chat/chat.go: handles RetryMsg — starts a fresh cancel context, resets stream tracking, shows the working spinner, and calls App.Retry. Guards against read-only sessions and in-flight turns.

Notable bug fixed during review

RunStream re-emits a UserMessageEvent for the trailing session message before StreamStarted whenever SendUserMessage is set. On retry this would duplicate the user bubble already on screen — or, when the tail is a tool/assistant message, render non-user content inside a spurious user bubble. App.Retry now suppresses any UserMessageEvent observed before StreamStarted, while still forwarding genuine mid-run user messages (steer / follow-up) that arrive afterward.

Testing

• New unit tests:
  • TestErrorMessageShowsRetryAffordance (message rendering)
  • TestMouseClickOnRetryLabelEmitsRetryMsg (click detection)
  • TestApp_Retry_SuppressesReEmittedUserMessage (re-emission suppression)
• task lint → 0 issues
• task test → all green