feat(chat): surface background streams as per-tab status dots

[sneumannb5]

What

Replace the floating StreamToasts overlay with per-tab status dots on each chat tab, so background streams surface
in-context. Adds ADR 0006. Closes #286

Why

Floating toasts stacked up and detached the stream state from the tab it belonged to. Per-tab dots put liveness where
the user is already looking.

Test plan

• npm run lint && npm run typecheck && npm run test in web-ui (0 errors, 173 tests pass)

Risk / blast radius

UI-only. No schema, API, or env-var changes. i18n de/en updated.

[ConnysCode]

Review: Request changes

Two things to change before merge — both non-cosmetic. The implementation itself is sound: I verified the removal is clean, the done-dismiss is safe, and i18n parity holds (details below).

Must change before merge

🟠 a11y — error vs done dots are distinguished by colour alone. All three dots share shape and size; running carries a non-colour cue (animate-pulse), but done (accent) and error (danger) differ only in hue. The aria-label/title cover screen-reader and hover users, but a colour-blind operator glancing at the tab strip can't tell "ready" from "failed" without hovering — and ADR-0006 elevates §8 ("colour is never the sole signal") to ship-blocking. Please give the error dot a non-colour cue (e.g. a ring, or a hollow-vs-filled shape) so the distinction survives without hover. Detail inline on ChatTabs.tsx:106.

🟠 The new state logic ships untested. tabStreamState and the done-only dismiss in handleSelect (chat/page.tsx:441) encode the subtlest part of this change — no dot on the active tab, aborted → no marker, forget only done while keeping error/aborted/running. The repo already unit-tests pure logic (29 test files, e.g. i18n-parity.test.ts). Please add a test covering the tabStreamState matrix (active / no-record / running / done / error / aborted) and the handleSelect dismiss rule before merge, so the contract ADR-0006 describes can't silently regress. Detail inline on ChatTabs.tsx:83.

Verified against the head clone

• Clean removal. No dangling references to StreamToasts or its streamToasts i18n namespace anywhere (grep across the head clone — only docs/** mention it now). No orphaned useTranslations('streamToasts').
• dismiss('done') on select is safe. Messages render from activeSession.messages and the liveness pill from message.liveness — both independent of the stream record. The only per-session store read on the active tab is agentUnavailableSlug (chat/page.tsx:547), which a done record never carries. Deleting the record on select breaks nothing inline; error / aborted / running are correctly kept so the recovery banner, stop button, and composer lock stay correct.
• i18n parity holds. streamToasts.* removed from both de.json and en.json; three chatTabs.* keys added to both with matching {title} placeholders, so i18n-parity.test.ts (key-set + placeholder equality) stays green.
• tabStreamState matrix is correct — active → no dot, aborted → no dot, running / done / error as documented in the ADR.

PR description — factcheck

Claim	Reality
"UI-only. No schema, API, or env-var changes."	✅ Only web-ui/** .tsx, messages/{de,en}.json, and docs/** touched.
"i18n de/en updated."	✅ Both locales updated symmetrically; parity test unaffected.
"0 errors, 173 tests pass."	Not re-run here (heavy Next.js install skipped). Consistent on inspection — no dangling refs, parity test stays green, types align.
"Closes #286."	✅ Option 2. The issue's illustrative "inline status row in the affected session" is already covered by the existing inline streaming UI + inline errors, per ADR-0006.

Nits (non-blocking)

• web-ui/app/_components/ChatTabs.tsx:215 — the dot's aria-label / title repeat the session title ("{title}: response ready") while the dot sits inside a tab that already exposes that title, so a screen reader announces the title twice. Consider dropping {title} from the dot label (just "response ready"); the tab context already supplies it.

(Reviewed against head 9665ed4.)

[ConnysCode]

error and done dots are distinguished by colour alone. All three dots share the same shape and size; running carries a non-colour cue (animate-pulse), but done (accent) and error (danger) differ only in hue. The aria-label / title cover screen-reader and hover users, but a colour-blind operator glancing at the tab strip can't tell "ready" from "failed" without hovering — and ADR-0006 itself elevates §8 ("colour is never the sole signal") to ship-blocking. Please give the error dot a non-colour cue too (e.g. a ring, or a hollow-vs-filled shape) so the distinction survives without hover — that is what makes the §8 claim in the ADR actually hold visually.

[ConnysCode]

The new state logic ships without tests. tabStreamState here (and the done-only dismiss in handleSelect, chat/page.tsx:441) encode the subtlest part of this change: no dot on the active tab, aborted → no marker, and forget only done while keeping error / aborted / running. The repo already unit-tests pure logic (29 test files, e.g. i18n-parity.test.ts), and both of these are pure and trivially testable. Please add a test covering the tabStreamState matrix (active / no-record / running / done / error / aborted) and the handleSelect dismiss rule, so the contract ADR-0006 describes can't silently regress.