fix(web): composer text and placeholder stay clear of the send buttons - #30568

Fix for the redesign stack, targets layer 2 (#28840).

Problem

Prompt text in the redesign composer could run under the paperclip and the mic/send pill. Line 1 kept a gutter for the buttons, but line 2 could run full width and sit right under the pill. Sometimes the buttons dropped to their own row instead. The placeholder ignored the buttons completely, so "for commands" was covered by the pill.

Fix

The buttons now stay in the top-right corner, and text wraps around them:

• A spacer sized to the buttons. The float beside the buttons is one line tall for a single line, so the composer doesn't grow. Once the prompt wraps, the float grows to the buttons' measured height plus a 6px gap (--spacing * 1.5). Every line beside the buttons keeps the gutter, and lines below them use the full width. This works for any font size, line height or button size, because it reads the real size of the buttons.
• Scrolling prompts. Past the 8-line cap, a float would scroll away and let text slide under the buttons. So an overflowing prompt gets a right margin the width of the buttons on every line, which also keeps the scrollbar clear. This mode only ever makes lines narrower, so switching in and out of it can't flip back and forth.
• Placeholder. It gets the same right padding, so its ellipsis lands before the buttons.

The observer no longer needs the hidden clone of the editor that it used to measure wrapping. It measures the paragraphs' own height, which ignores the float. The mobile resting state reads the same multi-line signal as before.

Before / after

Wrapped prompt — layer 2, 2× captures

Before	After
Layer 2, dark, 560 px
Layer 2, dark, 900 px
Layer 2, light, 560 px
Layer 2, light, 900 px

Empty placeholder with mic pill — layer 6, 2× captures

Before	After
Layer 6, dark, 560 px
Layer 6, dark, 534 px
Layer 6, light, 560 px
Layer 6, light, 534 px

Validation

• New observeComposerPromptLayout.browser.test.ts: wrapping, entering scroll mode, and shrinking back to one line. Fails on the layer, passes here.
• Checked by hand on layer 2 and on a port to layer 6, at composer widths of about 470–720 px, dark and light: single line, 4 wrapped lines, a long URL, the 8-line scroll, and the placeholder. No text rect touches the buttons. The first full-width line starts 15px below the pill.
• vp run check:changed origin/maria/redesign/2-composer-timeline passes.

Made with Claude Opus 5.5 in Claude Code.