fix(web): tighten the composer on narrow screens

[Antisophy]

The composer keeps its desktop spacing on a phone, where it takes a noticeable slice of the screen: 12px/16px around the box, 10px/14px inside the textarea, and buttons padded 10px/20px.

This halves that below 768px and gives the composer's buttons equal padding on all sides.

The textarea's min-height: 40px is one line's worth at the full padding, so it drops to 30px to match. Left alone it holds open roughly ten pixels of dead space under a single line, which disappears only once word wrap outgrows the floor, so the box looks correct while typing a long message and wrong while typing a short one.

Nothing outside the max-width: 768px block changes, so desktop layout is untouched.

[Antisophy]

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

• fix(web): tighten the composer on narrow screens #15 (this one): composer spacing
• fix(web): stop the page dragging sideways on narrow screens #16: the page dragging sideways
• feat(web): hide the composer while reading history on narrow screens #17: hiding the composer while reading history
• 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.