R8a: turn the composer into a floating island, not a full-width footer bar

[dovvnloading]

Problem

The composer was a <footer> spanning the full window width, docked flush at the bottom with no visual container of its own - a plain textarea and a row of individually-bordered buttons sitting directly in the app background. It read as a form embedded in a page, not a self-contained control surface.

Change

• Structurally, <footer className="app-composer-region"> is gone. The composer now lives inside <main className="app-canvas-region"> as a new absolutely-positioned layer, the same pattern already used for the token counter and notification banner, so the canvas renders full-bleed behind it.
• The card itself picks up the same elevated-surface language already used by the app's popovers/dialogs (node-body background, border, shadow, ~14px radius), centered with a responsive max-width so it never spans the full window.
• Inside the card: the textarea drops its own border/background and blends into the card surface (focus now highlights the whole card via :focus-within, not just the field); the attach/reasoning/model controls go flat and borderless until hover instead of each being its own bordered box; send/cancel break out into their own zone behind a single hairline divider, with send as the one circular, filled accent control.
• The card's bottom offset is derived from the token counter's actual measured footprint so the two never collide - verified directly via real bounding-rect measurements at both a 960px and a wide desktop window width, not just eyeballed.
• Deleted two fully orphaned CSS rules (.app-composer-region and .app-composer-placeholder, an unrelated dead leftover from an early pre-R2.3 scaffold) found sitting next to the selector this change repoints - neither had any remaining JSX reference.

No changes to Composer.tsx's element tree, aria-labels, or data-overlay-trigger attributes - this is a CSS/layout change only, so existing test coverage for composer behavior is untouched and still exercises the same DOM contract.

Test plan

• Full frontend suite: tsc --noEmit clean, lint 0 errors, 785 tests passed, build succeeds
• Full backend suite: 989 passed (unaffected - this is a frontend-only change)
• Verified in a live browser preview against the real running app: card renders as a bounded, centered island (760px cap, confirmed via getBoundingClientRect) at both 1189px and 960px window widths, with zero vertical overlap against the token counter's real measured position at either width
• Verified the reasoning/model popovers still render correctly, unclipped, at a short 600px window height