feat(site): add linear.svg icon (#26967)

> 🤖 This PR was written by Coder Agents on behalf of Jake Howell.

Adds a bundled `linear.svg` icon at `site/static/icon/linear.svg` so
template authors can reference Linear as a first-party `/icon/`.

`site/src/theme/icons.json` is regenerated by `make
site/src/theme/icons.json` — the diff is a single-line insertion between
`lakefs.svg` and `lxc.svg`.

## Verification

- `go test ./scripts/gensite -run TestSVGIconAttributes/linear.svg` —
PASS (`width="256"`, `height="256"`, `viewBox="0 0 256 256"`)
- `make lint/site-icons` — PASS
- `go test ./scripts/gensite -count=1` — PASS (full SVG attribute sweep)

site/src/theme/icons.json

@@ -87,6 +87,7 @@
 	"kiro.svg",
 	"kotlin.svg",
 	"lakefs.svg",
+	"linear.svg",
 	"lxc.svg",
 	"matlab.svg",
 	"memory.svg",

site/static/icon/linear.svg

@@ -0,0 +1,10 @@
+<svg width="256" height="256" viewBox="0 0 256 256" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g clip-path="url(#clip0_3_2)">
+<path d="M30.784 44.5867C42.7944 30.5877 57.6927 19.3541 74.4554 11.6577C91.2182 3.96128 109.448 -0.0156565 127.893 4.63209e-05C198.656 4.63209e-05 256 57.344 256 128.096C256 166.923 238.72 201.728 211.413 225.216L30.7947 44.5867H30.784ZM19.3813 60.0107L195.979 236.608C190.389 240.128 184.512 243.221 178.379 245.845L10.144 77.6214C12.7787 71.488 15.872 65.6 19.3813 60.0107ZM3.43467 97.7387L158.261 252.565C150.688 254.4 142.869 255.573 134.848 256L0 121.152C0.419464 113.258 1.56927 105.42 3.43467 97.7387ZM1.62133 149.6L106.4 254.389C80.2347 249.919 56.1019 237.438 37.3322 218.668C18.5624 199.898 6.08107 175.765 1.61067 149.6H1.62133Z" fill="#5E6AD2"/>
+</g>
+<defs>
+<clipPath id="clip0_3_2">
+<rect width="256" height="256" fill="white"/>
+</clipPath>
+</defs>
+</svg>