RFC: Cross-platform in-app toasts with avatars & privacy controls (Windows/Linux/macOS)

### Your use case

#### What would you like to do?
Have Element Web provide **in-app toast notifications** (small overlay inside the app), with:
- room/DM avatar (or branded fallback),
- granular privacy toggles: "Show sender info" and "Show message preview",
- consistent click-through to focus Element and open the room/thread,
- identical behavior across Windows / Linux (GNOME/KDE) / macOS.

#### Why would you like to do it?
System notifications behave differently per OS/DE (avatar support, grouping, click-through/focus).
Users also want predictable **privacy** (be able to hide sender/preview everywhere) and a modern,
compact UI that matches Element's look & feel.

#### How would you like to achieve it?
- **Element Web (this repo):**
- Add an in-app toast UI component (overlay) + event bus.
- Add device-level settings:
- *Use in-app toasts (instead of system)*,
- *Show sender info (like avatar) in desktop notifications*,
- *Show message preview* (enabled only if sender info is on).
- Privacy policy: when sender/preview are off, show brand title and a generic message ("You have a new message").
- **Element Desktop (Electron):**
- Wire click-through (toast -> focus app -> navigate to room/thread).
- Windows: ensure correct App identity (AUMID) and app icon/name when system toasts are used; in-app toasts are the primary path.
- **Cross-platform goals:**
- Parity for focus behavior and dismissal,
- Stacking/positioning (1–3 concurrent toasts), auto-dismiss, a11y (ARIA live + keyboard).


### Have you considered any alternatives?

- Keep system notifications only: inconsistent UX/privacy across OSes; limited avatar support.
- Fully native per-OS implementations: higher maintenance cost; the proposed in-app approach is simpler and consistent for a first step.

### Additional context

**Related**
- element-hq/element-desktop#2575

**Work in progress branches**
- [ogyrec-o/element-web@feat/notifications/in-app-toasts-crossplatform](https://github.com/ogyrec-o/element-web/tree/feat/notifications/in-app-toasts-crossplatform)
- [ogyrec-o/element-desktop@feat/notifications/in-app-toasts-crossplatform](https://github.com/ogyrec-o/element-desktop/tree/feat/notifications/in-app-toasts-crossplatform)

**Acceptance criteria (initial)**
- [ ] In-app toasts render with room/DM avatar or branded fallback
- [ ] Device settings: use in-app toasts, show sender info, show preview (depends on sender)
- [ ] Privacy respected: when off, no sender avatar/name and generic message text
- [ ] Click-through: toast -> focus Element -> open room/thread
- [ ] Basic stacking (1-3), auto-dismiss, a11y (ARIA live, keyboard)
- [ ] Windows base tested; Linux and macOS parity to follow

I'm happy to iterate with maintainers' feedback.