[Design] Redesign the chat composer

[MaggieAppleton]

Base branch: main
Branch: maggie/redesign-chat-composer
Depends on: None

Goal

Replace the visually inconsistent chat composer shell with a polished shared composition built from proven component patterns while preserving Chopin's collaboration behavior.

Context From Planning

• The composer in /design-audit#surfaces looks inconsistent with the rest of Chopin and should not be reinvented from scratch.
• Investigate shadcn Input Group and AI SDK AI Elements PromptInput as references, not preselected dependencies.
• AI Elements PromptInput is a close reference for auto-resize, submit-status icons, attachments, and keyboard behavior: https://elements.ai-sdk.dev/components/prompt-input

Current Behavior

The production composer owns acknowledged drafts, # document references, a listbox picker, send/stop state, keyboard submission, connection/read-only state, and inline send errors. Its visual shell is assembled from generic field/button classes and local layout utilities.

Desired Behavior

Create a cohesive composer with a clear input surface, footer/actions, reference affordance, busy/error presentation, and excellent keyboard/focus behavior. Reuse a library pattern where it fits, but adapt it to Chopin's existing state model.

Acceptance Criteria

• A short library evaluation compares shadcn Input Group, AI Elements PromptInput, and adapting the current composer; it records the chosen direction and dependency cost.
• The design audit covers empty, drafted, focused, disabled/disconnected, submitting, Planner-working/stop, reference-picker, reference-limit, and send-error states.
• Existing acknowledged-draft behavior, # references, picker keyboard navigation, Enter/Shift-Enter behavior, submit focus restoration, stop action, and error recovery remain intact.
• The composer is usable at narrow widths and with long referenced document names.
• The visual shell is shared where appropriate with comment/research composers without forcing their different domain behavior into one state machine.
• Maggie approves the audit specimen before production migration.

Verification Commands

• bun run types
• bun run ci
• bun test apps/web/src/chat
• bun run e2e

Primary Surfaces

• /design-audit#surfaces
• Chat composer in apps/web
• Reference picker and acknowledged composer drafts

Expected PR Checks

• Repo required checks

Report Requirements

• Library comparison and decision
• Before/after screenshots for key states and widths
• Keyboard/accessibility evidence
• Confirmation that draft/reference behavior is unchanged

Implementation Notes

• Treat external components as visual/interaction references. Chopin's wire protocol and draft reconciliation remain authoritative.
• Do not replace the composer with a component that assumes a different message or attachment model.
• Keep send errors associated with the composer without collapsing the input layout.

Stop Conditions

• A candidate library requires replacing Chopin's draft/reference state model.
• Attachment scope or new composer features would expand beyond this visual redesign.