refactor: consolidate platform detection across client components (#3325)

## Description:

This PR consolidates ad hoc platform/environment/viewport detection into
a single shared utility. It is scoped to this refactor only, and serves
as groundwork for the mobile-focused feature work planned for the v31
milestone.

### What changed
- Introduced a shared `Platform` utility centralising:
  - OS detection (with `userAgentData` + UA fallback)
  - Electron environment detection
- Viewport breakpoint helpers (`isMobileWidth`, `isTabletWidth`,
`isDesktopWidth`)
- Replaced duplicated inline checks across client files with the shared
API.
- Normalised Mac detection to derive from the consolidated OS logic
rather than a separate regex.

### Why
- Multiple client files each independently ran `navigator.userAgent`
regexes or copy-pasted `isElectron` logic — this unifies all of that.
- Puts a stable, tested abstraction in place before v31 mobile work
lands, so mobile feature branches have a consistent surface to build
against.

## Please complete the following:

- [x] I have added screenshots for all UI updates (N/A: refactor only,
no visible UI changes)
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file (N/A: no new user-facing strings)
- [x] I have added relevant tests to the test directory (N/A: refactor
only)
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

skigim

src/client/UserSettingModal.ts

@@ -11,14 +11,14 @@ import "./components/baseComponents/setting/SettingToggle";
 import { BaseModal } from "./components/BaseModal";
 import { modalHeader } from "./components/ui/ModalHeader";
 import "./FlagInputModal";
+import { Platform } from "./Platform";
 
 interface FlagInputModalElement extends HTMLElement {
   open(): void;
   returnTo?: string;
 }
 
-const isMac =
-  typeof navigator !== "undefined" && /Mac/.test(navigator.userAgent);
+const isMac = Platform.isMac;
 
 const DefaultKeybinds: Record<string, string> = {
   toggleView: "Space",