Permanent per-pair link for calendar events (#203)

[aleksejs1]

Closes #203, part of #213.

What

• /pair/{userIdA}/{userIdB} — a pair's permanent link. It redirects to the pair's open chain meeting (one-offs ignored), so it keeps working after the meeting is archived and the next cycle starts.
• No open meeting: a page with the last closed meeting's date, "Schedule the next one" (opens the create form with the colleague preselected) and "Open the last 1:1".
• A pair the user isn't part of, an unknown user, or no shared meetings: "No 1:1 behind this link", not a blank page.
• "Calendar link" button in the meeting page's header copies the link and shows a hint about pasting it into the recurring Google Calendar / Outlook event. Where the clipboard is unavailable, the link is shown in a field to copy by hand.

Frontend only: no new API, no schema change.

The issue's open questions

• URL format: the link names both people instead of the proposed /with/{counterpartUserId}. A counterpart-only link works for just the person who copied it, while a recurring 1:1 is normally one shared calendar event with one description. pairPath() sorts the ids, so both participants get the same link.
• Chains in both role directions: the server already matches a pair as unordered, so a pair has one chain whichever of the two leads it. The link ignores roles.

Beyond the issue

"Schedule the next one" preselects the colleague on the create form (an in-memory hand-over like "Create another"); UserTypeahead now shows a value set from outside.

Left open

Both are listed in docs/decisions/2026-10-03-pair-calendar-link.md:

• A pair whose chain is closed but who still have an open one-off see "No open 1:1 with …" with no pointer to the one-off.
• The preselected colleague replaces text typed into the colleague field before the form's first load finishes.

Verification

• Unit tests for pairMeeting() (open over closed, most recent closed, next cycle after an archive, one-offs ignored, unknown user), pairCounterpartId(), the route, and the create-form hand-over.
• A new dual-actor Playwright test follows the link through two cycles on the real e2e stack, as both participants. Full e2e suite: 51 passed.
• Typecheck, lint, format, knip, unit tests (837), duplication and doc-link checks are clean. make verify-pr-ready was not run in full: no backend change.
• Three code-review rounds: round 1 led to the two-id URL, round 2 found a redirect that could fire after the page was left (fixed), round 3 came back clean.

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