Fix mobile composer touch behavior

[Noojuno]

• Use coarse-pointer detection to switch Enter handling
• Increase composer text size on mobile
• Surface sidebar actions on touch devices
• Collapse composer on mobile by default; tap to expand

What Changed

Mobile touch UX improvements to the chat composer and sidebar:

• Enter behavior on touch devices: Bare Enter inserts a newline; Cmd/Ctrl+Enter sends. Desktop behavior (Enter sends, Shift+Enter newlines) is unchanged.
• Collapsed mobile composer: On small viewports, the composer starts collapsed as a single-line preview with an inline send button. Tapping anywhere on it expands the full editor. The composer re-collapses when focus leaves.
• Composer text size: Uses text-base on small screens to avoid iOS auto-zoom on focus, stepping down to text-[14px] at sm and above.
• Sidebar touch targets: The cloud icon is hidden on coarse-pointer devices and the new-thread button is always visible and tappable (no hover required).
• Send button focus preservation: All send/action buttons call preventDefault on pointerDown to prevent stealing focus from the editor on touch devices.

Why

On mobile/touch devices, the composer was hard to use: Enter would send instead of adding newlines (no physical Shift key), text was too small (triggering iOS auto-zoom), sidebar actions were hidden behind hover states that don't exist on touch, and the full composer took up too much vertical space on small screens.

UI Changes

Composer collapsed (mobile default):

• Single-line bar showing the current prompt text (or placeholder), with an inline send button.

Composer expanded (after tap):

• Full multi-line editor with all toolbars, same as desktop. Collapses again when focus leaves the composer surface.

Sidebar:

• New-thread button always visible on touch devices instead of requiring hover.

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