tui: split composer attachment and popup state

[etraut-openai]

Why

ChatComposer currently owns text editing alongside attachment bookkeeping and popup lifecycle state, while BottomPane still triggers a couple of popup resyncs after composer methods that already do that work internally. That blurs the ownership boundary and makes the composer harder to simplify safely.

This PR is part 1 of a two-part cleanup. It peels off the composer state that can move cleanly on its own, so the follow-up can tackle the heavier draft/editing boundary without mixing every concern into one diff.

What changed

• Move local and remote image bookkeeping, placeholder relabeling, and remote-image keyboard selection into AttachmentState.
• Move active-popup and popup-dismissal/query bookkeeping into PopupState.
• Update composer and history-search paths to use those state owners directly.
• Remove redundant BottomPane popup synchronization after paste handling and insert_str.

Part 2

The follow-up PR will finish the cleanup around the remaining composer boundary: split out the draft/editing-oriented state and footer/status presentation concerns that still live in ChatComposer, then revisit the leftover BottomPane pass-throughs once those ownership lines are explicit. The goal is for ChatComposer to coordinate a few focused collaborators instead of continuing to be the landing zone for every input-path concern.

Verification

Did manual smoke tests.

[fcoury-oai]

Did a smoke test with some composer features and it worked as expected. Code looks good as well.