fix(calendar): make URLs in the event description tappable

[lucletoffe]

The problem

A meeting invitation carries the join URL in the event description - Teams, Meet, Zoom and the plain iTIP fallbacks all do - and the event detail sheet renders it as bare <Text>. The one actionable thing in the event cannot be tapped: you have to select the URL by hand and paste it into a browser.

The mail body already linkifies (plainTextToSafeHtml, rendered in the WebView); only the calendar was left out. Same gap as bulwarkmail/webmail#968, which fixes the web client's three description renders.

The change

• src/lib/linkify-text.ts - splitTextLinks(text) splits plain text into runs and marks the http(s) URLs, deliberately using the same rule as plainTextToSafeHtml in lib/email-html (/(https?:\/\/[^\s<>"']+)/g), so a message linkifies identically in the body and in the calendar. The URL terminating at <> also means the <https://...> form mail clients emit yields a bare URL, not one with a trailing bracket.
• DetailRow gains an opt-in linkify prop, set on the description row only - never on rows the app composed itself.
• Each URL goes to openExternalUrl(url, { confirm: true }), the same guard the "Open link" button on the video row already uses: scheme allow-list plus the host confirmation, since this text arrives in someone else's data. The regex is only a hint about where a link starts and ends; open-url remains the thing that decides whether it may be opened.

Checks

• tsc --noEmit clean.
• vitest run: 972 passed, 14 skipped, 0 failures.
• New src/lib/__tests__/linkify-text.test.ts (6 tests): URL marked with surrounding text preserved, <https://...> termination, several URLs across lines, non-http schemes left alone (javascript:, file:, tel:), no characters lost when reassembling, and no lastIndex leak between calls on the module-level regex.
• Not exercised on a device build - the change is a straight parallel of the web fix, and the rendering path is a <Text> inside the existing DetailRow.