feat: harden support, localize knowledge and add operator availability [skip ci]

Source-only publication of reviewed commit 64e96a7f423673dc29ad56485be9fc79bce8af1c.

Passed in isolation: focused frontend/backend tests, race checks, TypeScript typecheck, lint, Go compilation, Flowgram build, and deterministic SDK build/tests.

Not accepted as a production release: Next.js export/native assembly remains blocked by environment memory limits; browser interaction and MySQL staging acceptance remain unverified. No production server, database/schema, runtime configuration, deployment, release tag, or workflow execution was performed.

Expected source tree: 0a4a7528e9.

docs/support-availability-ui.md

@@ -0,0 +1,65 @@
+# Support availability UI
+
+Modified in this independent fork; see `hardening/README.md`. Source-only feature,
+not a production deployment or a claim of browser acceptance.
+
+## Interfaces
+
+- System → Support availability: deployment-default IANA timezone and explicit
+  team calendar mode. A team can keep dated shifts or opt into recurring weekly
+  periods and date exceptions. Switching modes never deletes dated shifts.
+- Recurring periods support split shifts, overnight ends and `00:00`–`24:00`.
+  A closed date sends no open periods; unsaved hidden period inputs are retained
+  while toggling the date back to open. Draft preview calls the server evaluator.
+- Existing dated-shift screens explain that stored rows are inactive for teams
+  using recurring mode. They do not imply both schedules are combined.
+- Header/workbench availability controls persist Ready, Busy, Break, Unavailable
+  and a half-open vacation interval. The backend captures the return status when
+  vacation is first scheduled. Logging in does not reset an active absence.
+- Vacation and preview inputs explicitly use the browser timezone, show exact
+  UTC instants, reject nonexistent DST times, and expose a choice for repeated
+  local times. Recurring calendar results use the configured timezone. Opaque
+  legacy process timezone names render next-opening instants explicitly in UTC.
+- Admin profile rows show live presence, effective working status and server
+  eligibility separately. Dedicated status changes are independent of general
+  profile edits. The unused offline-message flag is retained but read-only and
+  labeled as legacy, never as permission to assign to disconnected staff.
+- Transfer candidates come from the server's conversation-scoped manual
+  eligibility endpoint, not a client-side `serviceStatus === Idle` guess.
+- Customer chat shows technical connection separately from aggregated human
+  support availability. No employee identity, absence interval or private reason
+  is sent by the public status endpoint. Scheduled opening copy does not promise
+  that an operator will be connected.
+
+## Safety and interaction coverage
+
+The API adapter uses the existing request/session envelope and bounds background
+availability reads to 10 seconds. Components discard unmounted/old-session
+responses. Mutations and post-confirmation callbacks are checked against the
+captured authenticated session. Saves are single-flight; failed saves retain
+inputs. Read-only/preview Enter does not submit calendar changes. Closing a dirty
+form, changing scope, and internal/browser navigation warn before discarding.
+
+The calendar is a scoped configuration form rather than a paginated resource, so
+it uses DashboardPage/Toolbar, react-hook-form, existing field/input/combobox and
+confirmation components. The existing agent CRUD/calendar components remain in
+place. All new copy is available in Russian, English and Chinese.
+
+## Checks performed
+
+- Full TypeScript `--noEmit --incremental false`: passed.
+- Focused availability helper/API tests: 13 passed.
+- Additional independent transport/locale edge tests: 10 passed, including
+  independent deadlines, retry recovery, EU DST and Lord Howe 30-minute overlap.
+- Focused lifecycle tests: 6 passed, including StrictMode effect restart,
+  duplicate saves, late session changes and confirmation after unmount.
+- Focused lint: no errors; existing agent avatar `<img>` warning remains.
+- Git whitespace check: passed.
+
+The lifecycle tests execute actual component handlers in an isolated hook
+harness; they are not rendered-browser tests. Browser launch was blocked by the
+execution environment. Desktop/narrow-width rendering, focus behavior, keyboard
+interactions, native date/time controls, and real Next.js Back/Forward handling
+still require browser acceptance. No live account, customer message, production
+database or deployment was used. The integration owner records combined build
+and broader regression results separately in `hardening/VALIDATION.md`.