mobile: the App listener's rejection reaches the catch

[kurktchiev]

initReminderSync and onAppActive both dropped the promise
App.addListener returns inside a block body:

import('@capacitor/app').then(({ App }) => {
  App.addListener('appStateChange', ({ isActive }) => { if (isActive) resync() })
}).catch(() => {})

When the App plugin isn't behind the bridge (a native project whose Capacitor
cap sync never ran, or a custom platform), addListener rejects
(UNIMPLEMENTED), and since the arrow's block body never returns that
promise, the .catch() right behind it never sees it. Every boot raised an
unhandled rejection.

Both are one-line fixes that return the promise instead of dropping it.

frontend 2 new tests in mobile.listener.test.js, run with npx vitest run --maxWorkers=2. They use a plain rejecting object standing in for the App
plugin rather than a vi.fn, because a spy attaches its own handlers to every
promise it returns, which counts as handling it and would hide this exact
leak. Checked against the old code first (both tests failed, catching the
plugin's rejection message as an unhandled rejection), then against the fix
(both pass). 9 total in the two mobile test files, all passing.

#391 (OIDC sign-in) rewrites onAppActive, and its version already returns
the listener's promise, so the two conflict there. Resolving takes #391's
onAppActive and this PR's initReminderSync change.

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