Refine composer layout and unify neutral workspace controls

[IchenDEV]

Outcome

The composer now provides more writing space with model, reasoning and permission controls directly visible, attachment/send actions grouped on the right, and checkout in an inset footer below. The model picker uses a vertical provider rail while retaining search and favorites.

The main input keeps its white surface with a fixed 0.5px neutral edge and soft shadow. Header, sidebar and neutral composer controls use consistent theme-muted gray; selection, warnings, recording and filled-submit states retain their visual meaning.

Change record

• Change: docs/sdlc/changes/2026-09-28-composer-reference-layout/intent.md

Verified in the actual renderer across light/dark, compact/document and narrow layouts. Relevant composer/provider/draft, header and sidebar tests passed, as did style lint, renderer lint, TypeScript, production build and documentation/SDLC gates. Native Core/provider execution and Windows rendering were not exercised.

Review and release

• Acceptance evidence covers the current diff, including rendered UI where affected.
• Required local checks passed; limitations are recorded in Verification.
• Merge authorization: pending.
• Release authorization: not requested.