Orbit: no way to search/find text in the chat history

**Reported via beta feedback (v0.5.1, macOS/arm64).**

> "It would be great to have search functionality in the chat. I wanted to go back to check what an acronym meant and could not!"

There's no find-in-chat affordance. As sessions get long -- and Orbit sessions run long, across many tool calls -- scrolling back to recover a definition, an id, a path, or a previous answer is the only option.

The ask is modest: a find/search box over the current chat transcript (Cmd/Ctrl+F-style), ideally with match highlighting and next/prev stepping.

Worth considering as scope is decided:
- search the current chat vs. across past sessions/analyses (the latter overlaps #357's recent-analyses picker)
- whether it searches rendered text only, or also tool-call output blocks (where ids/paths usually live -- likely where users actually want to look)

_(Beta tester feedback; details anonymized.)_

Start at the current chat transcript and tool-call output blocks, then review #357's recent-analyses picker to keep the search scope distinct. Decide whether search covers rendered text only or tool output, and whether it is current-chat only; done means a Cmd/Ctrl+F-style box supports matching, highlighting, and next/previous navigation.

Written by the indexing model from the issue text.