feat(web): hide the composer while reading history on narrow screens

[Antisophy]

iOS pins the composer above the on-screen keyboard, then lets the two drift apart as soon as the page scrolls: the box ends up floating over a gap between itself and the keyboard, and only a drag that starts inside the box puts them back together. The page cannot observe that drift, so there is nothing to correct against from script.

Scrolling up to read is also the moment when neither the composer nor the keyboard is wanted, so this leans into that: leaving the bottom of the message list hides the composer and blurs the field, which is what actually dismisses the keyboard. Returning to the bottom brings the box back.

Details worth knowing:

• iOS will not raise the keyboard again without a real tap, so returning to the bottom restores only the box; typing needs one tap. That is a platform constraint rather than a choice.
• Hiding is display: none on a component that stays mounted, so a draft in progress survives being scrolled away from.
• MessageList gains an onAtBottomChange callback, fired from the scroll listener it already runs for overflow-anchor, with 24px of tolerance so momentum scrolling landing a pixel off zero does not toggle it.
• Scoped to the same 768px breakpoint as the rest of the mobile layout: wider screens have room for the composer and no keyboard to dismiss, so nothing changes there.

[Antisophy]

One of five independent changes for using cydo on a phone, split apart so each can be judged on its own:

• feat(web): hide the composer while reading history on narrow screens #17 (this one): hiding the composer while reading history
• fix(web): tighten the composer on narrow screens #15: composer spacing
• fix(web): stop the page dragging sideways on narrow screens #16: the page dragging sideways
• feat(web): attach images with a file picker, not just paste and drop #18: attaching images with a file picker
• feat(history): replay a window of a task's history, with older messages on demand #19: replaying a window of history, with older messages on demand

They touch different code and can merge in any order. #15 and #17 both add rules inside the same @media (max-width: 768px) block, so whichever lands second may need a trivial context rebase; nothing semantic overlaps.