lnkiai/m3e-canvas

Stage 1 is in: #9 is merged, thank you. On top of it, 435e63b adds what came out of trying it:

- Switching a screen lays it out again by the tidy rules (bars on their edges, the FAB in its corner, the rest flowing from the top) and moves the screens to its right over, so nothing lands outside or overlaps. The view stays put.
- Every screen keeps the bezel; desktop only has flatter corners. The switch sits in the label in place of the icon.
- The prompt target follows the screens (web once a desktop screen exists) unless the author picked one, and the target line and screen headings are written per viewport in all three languages.
- The preview cross-fades between screens of different sizes and no longer rescales mid-swipe.

Next, stage 2: the navigation rail and the desktop-specific layout rules. We will take that here.

Thanks, this is a good fit for where the project is going. Yes, let's do it, in stages.

**Model:** a frame has one size. Presets to start: phone 412×892 (current) and desktop 1280×800; tablet and custom later. Frames of different sizes can coexist in one document; there is no linking between a phone and a desktop version of a screen. To get both, duplicate the screen and switch it. The prompt treats same-named screens as one screen at two widths, and defaults the target to web when a desktop frame exists (an explicit Android target treats desktop frames as a landscape tablet).

**Where the switch lives:** a connected button group on the frame label (and in the frame inspector), not a global setting.

**Stages:**

1. Size on the frame. Add `w` / `h` to `Frame` (missing = phone, so old files keep working) and route every `PHONE_W` / `PHONE_H` use through it: `frameRect`, frame rendering (no phone bezel on desktop, a plain rounded rectangle), snapping, fit-to-screen, new-frame placement,...