feat(web): recall sent prompts with ArrowUp in the composer

[sethwebster]

Problem

Re-running or editing a recent prompt requires copying it from the timeline. Terminal-style ArrowUp and ArrowDown history would make this faster without adding another control.

The earlier request in #1777 is closed and locked. Its linked implementation, #4336, is still open, no longer applies cleanly to the current Lexical composer, and has three unresolved review findings.

Smallest useful behavior

• ArrowUp on an empty composer recalls the latest sent prompt in the active thread.
• Further ArrowUp presses move backward through sent prompts.
• ArrowDown moves forward. Moving past the newest entry restores the unsent draft.
• Editing a recalled prompt does not change the stored history entry.

Interaction constraints

• Normal ArrowUp and ArrowDown editing continues inside multiline prompts.
• Autocomplete menus keep their existing arrow-key behavior.
• History does not run during IME composition, modified key presses, approvals, or pending user input.
• History entries include the prompt text that the user saw, not terminal, element, preview, review, or provider-injected payloads.
• The just-sent optimistic prompt is immediately available.
• Switching threads does not replace that thread’s unsent draft with recalled text.

Scope

This is a web composer change. Desktop inherits the web client. Mobile has no hardware-arrow interaction to change. No provider or wire contract changes are needed.

Related: #1777 and #4336.