feat(stream): add an 'add to calendar' (.ics) action for stream end dates (#566)

[jarik2014]

Closes #566

Adds an "Add to calendar" action on /stream/[id] so a recipient can drop the
stream's end date into their own calendar instead of retyping it.

What's here:

• lib/calendar.ts — builds the .ics text by hand: RFC 5545 CRLF line endings,
  escaping for , ; \ and newlines, and line folding at 75 octets with a leading
  space on continuation lines. buildStreamIcs() takes the stream id, end date, token
  symbol, contract id, start date and the app URL and emits a single VEVENT with
  DTSTART/DTEND at the completion instant.
• components/stream/AddToCalendarButton.tsx — a small button rendered next to the end
  date. It builds the file in the browser and downloads it via a Blob + object URL, so
  no server round trip and no new dependency. It is disabled while the end date is
  missing and cleans up the object URL after the click.
• app/stream/[id]/page.tsx — renders the button with the data the page already has
  (StreamTimeline shows the same date), plus the two new mocks needed for the tests.
• Tests: lib/calendar.test.ts covers the escaping, the folding (unfolds the output
  before matching, since a long description is legitimately split), the exact byte
  layout of the header, and the round trip of a real end date; the component test drives
  the download path with a stubbed URL.createObjectURL; the page test asserts the
  button appears with the stream's end date.

Verification on this branch:

$ npx vitest run lib/calendar.test.ts components/stream/__tests__/AddToCalendarButton.test.tsx "app/stream/[id]/__tests__/page.test.tsx"
 Test Files  3 passed (3)
      Tests  44 passed (44)

$ npm run typecheck
components/stream/__tests__/StreamFlowChart.test.tsx(294,12): error TS1005: ';' expected.
components/stream/__tests__/StreamFlowChart.test.tsx(296,6): error TS1128: Declaration or statement expected.
components/stream/__tests__/StreamFlowChart.test.tsx(309,1): error TS1128: Declaration or statement expected.
components/stream/__tests__/StreamFlowChart.test.tsx(309,2): error TS1128: Declaration or statement expected.

Those four are pre-existing and unrelated: they sit in a test file this branch does not
touch, and they are parse errors in a file whose content is identical to main. No
errors in any of the files added here.

Two judgement calls worth flagging:

• The description folds at 75 octets, so the "Stream started / Contract / URL" lines are
  split mid-word in the raw file. That is what RFC 5545 asks for and Google/Apple
  Calendar unfold it, but if you would rather keep the text readable in a terminal I can
  shorten the description instead.
• The download is a Blob URL rather than a route handler, which keeps the change
  client-side. If you prefer a /api/streams/[id]/ics endpoint (so the link is
  shareable), say so and I will move it.