Compound adoption

## Overview

Meta-issue to help discuss and capture decisions on:

* Element Web adopting Compound Web to further usability and accessibilty targets *reliably*, reducing risk of future regressions
* Using Element Web to guide real-world Compound Web component scope and implementation
* Scrutinising & iterating on Element Web in a customer & strategically informed way - reducing complexity and time to market on execution

This issue aims to provide a lens to cross-team collaboration and unknowns, complimenting:

1. [Compound components board](https://github.com/orgs/vector-im/projects/52/views/18).
2. [Internal project tracking sheet](https://docs.google.com/spreadsheets/d/1GXI0i0A4PQhaw3FVpeR0acHE1wlsnHRGvwTLL9WpenM/edit?usp=sharing)

## Adoption plan

We'll port Element Web area by area as means to (1) guide component scope and implementation with real-world considerations (2) provide a harmonious enough app-wide experience for users while we make progress and ship (3) provide focus (4) align critical thinking and context switching with other teams and projects:

[The project is being managed in this sheet](https://docs.google.com/spreadsheets/d/1GXI0i0A4PQhaw3FVpeR0acHE1wlsnHRGvwTLL9WpenM/edit?usp=sharing)

We will have milestones (below). Each milestone will have phases. We may chose to move onto the next milestone without completing all phases of the current milestone. Please reference the sheet for more info and decision tracking.

```[tasklist]
### Tasks
- [ ] https://github.com/vector-im/element-web/issues/25546
- [ ] https://github.com/vector-im/internal-planning/issues/438
- [ ] Timeline: ...
- [ ] Composer: ...
- [ ] Room list & Space panel: ...
- [ ] Settings: ...
- [ ] Interstitial journeys: ...
```

## Known unknowns

A list of cross-team unknowns to work through as we adopt by area:

```[tasklist]
### Tasks
- [ ] https://github.com/vector-im/internal-planning/issues/432
- [ ] https://github.com/vector-im/internal-planning/issues/434
- [ ] https://github.com/vector-im/internal-planning/issues/435
- [ ] https://github.com/vector-im/internal-planning/issues/436
- [ ] https://github.com/vector-im/internal-planning/issues/444
```