Sign in on a phone with no console work, in the browser's own session

Mobile OAuth reused the desktop client all along; what stopped it was the
browser. Sending the user out to Safari or Chrome backgrounds the app, iOS
suspends it, and the redirect carrying the code arrives at a socket nobody
is accepting on. The consent page now opens in front of the app instead, in
SFSafariViewController or a Chrome Custom Tab, so the loopback listener
stays live and the existing `installed` client is enough. Verified against
Google's real consent screen on a simulator and an emulator.

A per-platform client is still supported and is now an upgrade rather than a
prerequisite. On iOS it buys ASWebAuthenticationSession, which shares
Safari's session so nobody is asked to sign in to Google twice. Android
needs nothing: Custom Tabs share Chrome's cookies, measured rather than
assumed. iOS session sharing could not be confirmed on the simulator and
wants a real device.

Never an app-owned WebView: Google blocks it, and rightly, since a webview
the app controls can read the password typed into it.

Cancelling is no longer reported as a failure. AuthEvent carries a
`cancelled` flag, set by comparing against the constant every back-out path
returns, and Google's `access_denied` on desktop counts too.

Five frontend bugs found by driving the real UI, not by reading it: the
details card slid under the tab bar leaving its buttons unhittable; the
ghost click after a touch pressed a button in the card that tap had just
opened, opening the editor by itself; the swipe that pages the day was dead
over every read-only block; 84px of macOS traffic-light lane was reserved on
platforms with no traffic lights; and the desktop header ignored the top
safe area on an iPad. A first launch now says what to do next rather than
showing an empty grid, and accounts are named as Google accounts throughout.

tests/overlays.spec.ts

@@ -17,12 +17,12 @@ const PANELS = [
   { name: "Calendars", open: openCalendars, contains: /you@example\.com/ },
   { name: "Settings", open: openSettings, contains: /Week view/ },
   {
-    name: "Accounts",
+    name: "Google accounts",
     open: async (page: Page) => {
       await openSettings(page);
       await page.getByRole("button", { name: "Manage" }).click();
     },
-    contains: /Connect an account/,
+    contains: /Connect a Google account/,
   },
 ];
 
@@ -71,7 +71,7 @@ test("Escape unwinds one layer at a time", async ({ page }) => {
   await openSettings(page);
   await page.getByRole("button", { name: "Manage" }).click();
 
-  const accounts = page.getByRole("dialog", { name: "Accounts" });
+  const accounts = page.getByRole("dialog", { name: "Google accounts" });
   await expect(accounts).toBeVisible();
 
   await page.getByRole("button", { name: "Disconnect" }).first().click();