feat(dashboard): dismissible system notice banner for every signed-in user (#2464)

* feat(dashboard): dismissible system notice banner for every signed-in user

Operator-set banner ("high load right now, some pages may respond slowly
or fail") rendered under the dashboard chrome for every signed-in user
while NEXT_PUBLIC_SYSTEM_NOTICE_UNTIL (ISO timestamp with offset) is in
the future. Closing it stores the deadline in localStorage, so each
browser sees it once; the banner hides itself at the deadline in open
tabs and is not rendered at all after it.

Why the problem occurred: there was no way to tell every user something
about the system itself. The existing banners are all per-company state
(sandbox, seat grace), so an operator notice had no home.

What was removed or simplified instead: no notices table, no migration,
no admin UI. One public env var carries both the on/off switch and the
expiry, and the same value is the dismiss key, so a later notice re-shows
once without any code change. No DB read, which matters because the
first use is a DB restart window.

Why this over the proposed shape: the request was a banner "until 23:00
tonight". Hardcoding that in code would need a second PR to switch off
or reuse; a DB-backed notice would read the database that is about to
go down. The env var expires on its own, and unset means gone.

Fixes #2463

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fs9PfHL7KpdidvUdxVkHXF

* fix(dashboard): system notice survives long deadlines, blocked storage, and every layout shell

Skeptic findings on 839a255f3:
- setTimeout clamps delays above 2^31-1 ms to ~1 ms, so a deadline more
  than 24.8 days out hid the banner instantly. Wait in bounded steps and
  re-check the clock.
- window.localStorage is a throwing property access when a browser blocks
  site data; read it behind a try so the dashboard never crashes over a
  notice.
- The close button was a hand-rolled 22px icon button; design.md requires
  the shadcn icon Button (40px target).
- The byrå-consultant shell and the stale-cookie shell rendered no banner,
  so "every signed-in user" was not true. The banner is now computed once,
  before the shell branches, and mounted in all three.

Refs #2463

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fs9PfHL7KpdidvUdxVkHXF

* fix(dashboard): system notice deadline requires a UTC offset

A date-time without Z or a numeric offset parses as local time, which is
UTC on Vercel and the operator's zone locally, so the same value would
mean different instants. Reject it instead (CodeRabbit on #2464).

Declined: scoping the dismissal key by user id. The notice is about the
system, not the account; per-browser dismissal is the sandbox banner's
semantics and keeps identity out of layout chrome.

Refs #2463

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fs9PfHL7KpdidvUdxVkHXF

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

.env.example

@@ -50,6 +50,11 @@ GOOGLE_MAIL_CONNECT_COMPANY_IDS=
 # NEXT_PUBLIC_SESSION_TIMEOUT_FORCE_ALL=false
 # SESSION_TIMEOUT_SECRET=
 
+# One-off system notice ("high load right now") shown once per browser to
+# every signed-in user until this ISO timestamp (include the UTC offset).
+# Unset or past: no banner. A new timestamp shows the banner once again.
+# NEXT_PUBLIC_SYSTEM_NOTICE_UNTIL=2026-09-10T23:00:00+02:00
+
 # Self-hosted only: set to true when public signup is turned off in your
 # GoTrue/Supabase auth config (GOTRUE_DISABLE_SIGNUP / "Allow new users to
 # sign up" off). GoTrue offers no clean server-side read of that setting, so