No Stop button: #25's stream-cancel backend has no frontend wire-up — users can't stop a streaming response

[matthewod11-stack]

Description

Issue #25 shipped the full backend cancel apparatus (StreamRegistry, CancellationToken, connection-drop-stops-billing, cancel_stream command), but the frontend never wired it up — there is no Stop button, and the frontend doesn't pass a stream_id to send_chat_message_streaming (the command comment explicitly calls this out as a pending follow-up). Users watching a long/wrong response stream in have no way to stop it, and abandoned streams bill to completion. Found during #112 manual QA (2026-07-09): the cancelled-audit-row path could not be exercised from the UI.

Current State

• Backend complete: chat::StreamRegistry + cancel_stream command (src-tauri/src/commands/chat.rs) — cancelling drops the upstream connection and (post-Audit log has one write site (interactive chat, frontend fire-and-forget) — backend LLM egress writes zero audit rows #112) writes a cancelled partial audit row
• send_chat_message_streaming accepts optional stream_id; when absent it generates a UUID the UI can never learn (see comment in src-tauri/src/commands/chat.rs)
• Frontend: sendChatMessageStreaming (src/lib/tauri-commands.ts) does not send streamId; no Stop affordance in the chat UI
• Audit log has one write site (interactive chat, frontend fire-and-forget) — backend LLM egress writes zero audit rows #112's integration tests cover the cancel path end-to-end (chat.rs: trial_stream_cancel_writes_cancelled_audit_row)

Suggested Fix

• Frontend generates a UUID stream_id per send; pass through sendChatMessageStreaming
• Show a Stop button while streaming (replace/augment the send button); onClick → cancel_stream(stream_id)
• Also call cancel_stream on conversation switch/unmount (the command is safe on unknown ids)
• Frontend handles the existing chat-stream-cancelled event (reset streaming UI state)
• vitest: pure-function tests for any extracted stream-state logic; manual QA confirms a cancelled audit row lands

Verification

• cargo test + npm test pass
• Manual: start a long response, hit Stop → stream halts, UI resets, audit_log gains a cancelled row with partial char count

Automation Hints

scope: src/contexts/ConversationContext.tsx, src/lib/tauri-commands.ts, src/components/chat/
do-not-touch: src-tauri/ (backend is complete)
approach: extract-and-move
risk: low
max-files-changed: 6
blocked-by: none
bail-if: requires backend signature changes beyond making stream_id required

Priority

Medium — user-facing control gap + token cost on abandoned streams; also unblocks manual QA of the cancelled-row path.

[matthewod11-stack]

Orchestrator opened PR: #149

Frontend-only wire-up of the existing backend cancel apparatus. tsc --noEmit clean, vitest 63/63 (+7). src-tauri/ untouched. ⚠️ Manual QA of the cancelled-audit-row path still needed before merge (can't be run unattended).

[matthewod11-stack]

Manual QA (the acceptance item PR #149 flagged as not-run) completed 2026-07-15 on the merged code, dev app:

• Stop mid-stream: stream halted, partial text retained, no error banner/retry chip, input reverted to Send — passed (Matt).
• Cancel on conversation switch: silent stop, no error — passed (Matt).
• Audit evidence: 3 status=cancelled rows in audit_log ([STREAM_CANCELLED after 0/415/427 chars], source=interactive) — partial billing cutoff recorded per Audit log has one write site (interactive chat, frontend fire-and-forget) — backend LLM egress writes zero audit rows #112.

Note: review found and fixed one bug pre-merge — the backend's ChatError::Cancelled invoke rejection fell through categorizeError and decorated the partial message with a generic error + retry chip. Fixed in 49a889c (isCancelledError + quiet-finalize catch branch).