fix(web): scope Composer drafts to Sessions - #480

Problem

Web Composer drafts are local to the component instead of the selected Session. Switching Sessions can show the previous Session's draft, and a late successful send receipt clears newer text typed after the send. Fixes #457.

Value

Users no longer lose new input when a send settles late, and drafts from one Session cannot appear in another Session. Failed sends remain editable.

Approach

• Pass the store's selected Session path into Composer as the draft scope.
• Clear drafts when the Session or new-workspace scope changes.
• Track draft revisions so a send only clears the exact unchanged draft it submitted.
• Transfer a pending new-workspace draft to the created Session until its first send is accepted.
• Add focused regressions for delayed and retyped sends, failed sends, Session changes, late receipts, and new-Session creation.

Validation

• bunx vitest run tests/web/app-render.spec.ts — 17 passed.
• bun run check — passed.
• git diff --check — passed.
• bun run test — run; two unrelated existing/environment-sensitive failures remain: the Windows Git process test reports -1 instead of 7, and the detached launch-card repaint test fails. The focused Web Composer suite passes.

Impact

• User-visible behavior: drafts are Session-scoped and newer edits are preserved.
• Model-visible context/tools: None.
• Runtime/lifecycle: None.
• Persisted config/data: None; drafts remain in memory and are not written to localStorage.
• Compatibility/risk: limited to the Web Composer's local draft lifecycle.