Meeting list: banner for a meeting open longer than its period (#204)

[aleksejs1]

Closes #204. Part of #213. Frontend only: no backend, schema or crypto change.

What changes

• List banner. AnketaList.svelte shows one line above the list for each meeting open longer than its period: "{name}: the 1:1 on {date} isn't closed yet." with a "Close and schedule the next one" link. The link goes to the meeting page's archive form through the follow-up email's existing #close fragment, so there is no new landing code.
• Suggested next date. For such a meeting the archive form defaults to the nearest date on the pair's cadence that isn't in the past (the meeting date plus a whole number of periods).
• Rule and date. Two pure functions in frontend/src/anketa/longOpen.ts: isOpenPastPeriod() and nextCadenceDate().
• Locales. One new string, anketaList.longOpenText, in all 6. The link text reuses anketa.closeAndScheduleNext.
• Docs. docs/user-flow.md, docs/history.md, CLAUDE.md.

Where this differs from the issue text

• The cadence date is the archive form's default for a long-open meeting however the page was opened, not only from the banner. Every other meeting keeps today plus one period.
• The banner text names the counterpart, since several meetings can be listed at once.
• A meeting whose counterpart was deleted gets no banner, nor does a one-off or a legacy meeting without a period.
• "Longer than its period" is strict: no banner exactly one period after the meeting date, one from the next day.

Known gap, not fixed

The banner can show for a meeting the viewer can't open because it is sealed under their previous keypair (after a password reset). The link then lands on the stale-key error page, the same as the ordinary row link. The list response has no field for this, so hiding the banner would need a backend change.

Verification

• Unit tests for both functions (longOpen.test.ts); the full suite passes (851 tests).
• A new e2e test in dual-actor-anketa.spec.ts, run against the e2e stack: banner, link, focus on the archive form, next date prefilled. The rest of the e2e suite was not re-run locally.
• Typecheck, eslint, prettier, knip, duplication and doc-link checks are clean.
• One independent code-review pass: no blocking findings, the gap above was its only note.

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