T24a: Agent, tool calls

- ToolCallCard: a step mark, the tool in mono, what it touched and how long
  it took; a press opens its arguments and result as code, an error in red
- ToolCallGroup folds several calls under one line
- ToolApprovalDialog over overlays::Dialog: Deny or Run, one answer; Escape,
  the scrim and the close deny
- PermissionPrompt: deny, always allow or allow once, in place
- chat::StepState gains Failed; chat::step_mark draws a step's mark for
  search steps and tool calls alike
- typography::format::took reads ms, seconds or minutes
- gallery: an Agent page with calls in each state, a group, the dialog and
  the prompt

src/chat/mod.rs

@@ -37,6 +37,7 @@ pub use play::{AudioMessage, VideoMessage};
 pub use projects::{
     ConversationExport, Project, ProjectKnowledgePanel, ProjectList, SharedConversationView,
 };
+pub(crate) use search::step_mark;
 pub use search::{DocumentChunkPreview, SearchProgress, StepState, WebResultCard};
 pub use status::{
     ErrorMessage, FeedbackForm, RateLimitNotice, ThinkingBlock, ThinkingDuration, ThinkingIndicator,