Preview: make "Fill panel" a real viewport option, and add aspect-ratio presets

[praneeth-red]

Problem or use case

Sizing the in-app browser feels clunky. Right now I either type a width and height by hand, or pick from a long list of 17 devices (iPhone SE, Nest Hub, Galaxy Z Fold 5, Surface Duo…). Most of the time I just want the page to fill the panel and follow it when I drag the panel bigger or smaller.

Fill mode already exists, but the only way back to it is the X on the device toolbar. That X looks like "hide this toolbar", not "fill the panel", so it's easy to get stuck on a fixed size.

Proposed solution

1. "Fill panel" as the first choice in the viewport dropdown, and the default. The page uses all the space the panel has and resizes live as I drag the panel.
2. Aspect-ratio presets: 16:9, 4:3, 1:1, 3:4, 9:16. Each one picks the largest size with that shape that fits the panel, and keeps the shape when the panel changes size.
3. A shorter device list. Keep a few common ones (one iPhone, one Android, one iPad) and put the rest under "More devices".

Why this matters

Most of the time I want either "use all the space" or "show this shape". Neither should mean typing in pixels or scrolling through a long list of devices.

Smallest useful scope

Point 1 alone. Points 2 and 3 can come later.