chore: clear pre-commit eslint warnings (unused symbols, stale disables, test any)

Cleans up the lint warnings the pre-commit hook surfaces, without any
behavioral change:

- Remove unused imports/vars/destructured props (parseISO, useEffect,
  format, durMin, roles, daysInYear, ALLOWED_PLUGIN_FILES, continuesBefore,
  isPushConnected, isSelected) and the now-unused parseDuration import.
- Drop three stale `// eslint-disable-next-line no-undef` directives that
  no longer suppress anything (browser-navigation, smime/crypto-engine).
- recurrence-expansion.test.ts: replace 39 `as any` casts with a cast-only
  `rule()` helper for partial recurrence-rule fixtures, typed access to
  utcStart/utcEnd (now on CalendarEvent), and the source's
  `Partial<CalendarEvent> & { excluded?: boolean }` for the excluded
  override. No defaults are injected, so the expansion logic sees the same
  partial rules as before (35 tests still green).

Remaining: 7 react-hooks/exhaustive-deps warnings are left as-is — adding
the missing deps changes effect/memo timing and needs per-hook review, not
a mechanical fix. tsc --noEmit clean; eslint 0 errors / 7 warnings.

lib/browser-navigation.ts

@@ -67,7 +67,6 @@ export function getPathPrefix(locale?: string): string {
  *   // Browser at /webmail/en/inbox  → /webmail/api/jmap
  *   // Browser at /en/inbox          → /api/jmap
  */
-// eslint-disable-next-line no-undef
 export function apiFetch(input: string, init?: RequestInit): Promise<Response> {
   if (input.startsWith('/') && !input.startsWith('//')) {
     return fetch(getPathPrefix() + input, init);