chore: add devcontainers icon (#24478)

Adds the devcontainers icon sourced from [Microsoft Fluent UI
`ic_fluent_cube_32_filled`](https://github.com/microsoft/fluentui-system-icons/blob/78c9587b995299d5bfc007a0077773556ecb0994/assets/Cube/SVG/ic_fluent_cube_32_filled.svg).
The registry module `devcontainers-cli` already references
`/icon/devcontainers.svg` — this adds the missing icon.

## Changes
- `site/static/icon/devcontainers.svg` — new icon
- `site/src/theme/icons.json` — auto-generated entry
- `site/src/theme/externalImages.ts` — registers icon as `monochrome` so
it renders correctly on both dark and light themes

> 🤖 This PR was created with the help of Coder Agents, and needs a human
review. 🧑💻

---------

Co-authored-by: Jake Howell <jake@hwll.me>

site/src/theme/externalImages.ts

@@ -152,6 +152,7 @@ export const defaultParametersForBuiltinIcons = new Map<string, string>([
 	["/icon/container.svg", "monochrome"],
 	["/icon/copyparty.svg", "blackWithColor"],
 	["/icon/database.svg", "monochrome"],
+	["/icon/devcontainers.svg", "monochrome"],
 	["/icon/docker-white.svg", "monochrome"],
 	["/icon/folder.svg", "monochrome"],
 	["/icon/gemini-monochrome.svg", "monochrome"],

site/src/theme/icons.json

@@ -40,6 +40,7 @@
 	"dcv.svg",
 	"debian.svg",
 	"desktop.svg",
+	"devcontainers.svg",
 	"discord.svg",
 	"do.png",
 	"do.svg",

site/static/icon/devcontainers.svg

@@ -0,0 +1,3 @@
+<svg width="256" height="256" viewBox="0 0 256 256" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M110.769 22.0568C121.86 17.791 134.14 17.791 145.231 22.0568L224.615 52.5894C233.884 56.1542 240 65.0591 240 74.9896V181.012C240 190.942 233.884 199.847 224.615 203.412L145.231 233.945C134.14 238.21 121.86 238.21 110.769 233.945L31.3845 203.412C22.1159 199.847 16 190.942 16 181.012V74.9896C16 65.0591 22.1159 56.1542 31.3845 52.5894L110.769 22.0568ZM75.1534 80.6472C71.0924 78.9065 66.3894 80.788 64.6489 84.8488C62.9085 88.9096 64.7897 93.6128 68.8507 95.3536L120.002 117.275V168C120.002 172.418 123.584 176 128.002 176C132.42 176 136.002 172.418 136.002 168V117.275L187.154 95.3536C191.214 93.6128 193.096 88.9096 191.355 84.8488C189.614 80.788 184.912 78.9065 180.85 80.6472L128.002 103.296L75.1534 80.6472Z" fill="#212121"/>
+</svg>