chore: drop dead IE11 MSStream check in iPadOS detection

[puckey]

Summary

The !window.MSStream clause in the iPadOS detection branch was imported from a circa-2013 Stack Overflow snippet that used it to exclude Internet Explorer on Windows Phone from being detected as iOS. It was never reachable under the `platform === 'MacIntel'` branch — IE never ran on macOS — so the guard was dead the day it was added. IE itself has been EOL since June 2022.

Removing it also drops the `@ts-expect-error` comment it justified (since `window.MSStream` isn't in the DOM lib types).

Bonus: comment upgrades

• One-line note on why `navigator.platform` (deprecated per MDN) is still in use here: `navigator.userAgentData.platform` is Chromium-only and doesn't expose the iPadOS-as-MacIntel masquerade.
• Acknowledge a known forward-looking limitation: if Apple ever ships a touchscreen Mac, it will also match `MacIntel + maxTouchPoints > 0` and be misclassified as iPad here. No such device exists today; revisit when/if it ships. (`navigator.standalone` was considered as a Touch-Mac-proof alternative, but I verified on macOS Safari today that `typeof navigator.standalone === 'boolean'` — Apple extended it to macOS at some point, so it can't be used to discriminate Mac from iPad anymore.)

Diff

-  // Workaround for ipadOS, force detection as tablet
+  // Workaround for ipadOS, force detection as tablet. `platform` is
+  // deprecated but remains the only reliable way to distinguish iPadOS
+  // desktop-mode (MacIntel + touch) from a real Mac on Safari/Firefox.
+  // Known limitation: if Apple ever ships a touch-screen Mac, it will
+  // also match `MacIntel + maxTouchPoints > 0` and be misclassified as
+  // iPad here. No such device exists today; revisit when/if it ships.
   // SEE: https://github.com/lancedikson/bowser/issues/329
   // SEE: https://stackoverflow.com/questions/58019463/how-to-detect-device-name-in-safari-on-ios-13-while-it-doesnt-show-the-correct
   const isIpad =
-    platform === 'iPad' ||
-    // @ts-expect-error window.MSStream is non standard
-    (platform === 'MacIntel' && maxTouchPoints > 0 && !window.MSStream);
+    platform === 'iPad' || (platform === 'MacIntel' && maxTouchPoints > 0);

Test plan

• `pnpm test` — all tests pass
• `pnpm lint` passes
• `pnpm format:check` passes
• `pnpm build` succeeds