feat(meta): colourise the plugin artwork and say what it actually is

Two things the Plugins list made obvious once the icon rendered beside the
built-ins.

The artwork was flat: one blue panel, one darker blue rail. The built-ins
each carry their own two-hue palette — the file header says so outright
("these glyphs carry their own brand colors and gradients instead of riding
currentColor") — and at the 30px a row renders at, a single-hue mark reads as
a plain block. Three candidates were rendered next to the real built-in
artwork at the real sizes (30 / 36 / 72, light and dark) before picking one: a
violet rail sat too close to the panel's deep blue to read as a second colour,
and a pale teal panel washed out. The shipped pair is blue for the panel
(gradient) and teal-green for the rail (gradient), which stays legible at row
size and matches the Subagent artwork's proven blue/green combination without
sharing its silhouette.

The description sold the plugin as someone else's sidebar. It now leads with
what it is — better — in one sentence rather than a colon and a feature list,
in both locales, and both `package.json` (the npm blurb and the list's
fallback label) and `dsh.plugin.json` say the same thing. The README never
framed it that way, so nothing there changed.

The artwork guard gained the two properties this decision rests on: the mark
must be colourised (two gradients, no currentColor) and must not use a
CSS-only paint, since the icon ships as an <img> and the search artwork's
foreignObject technique would stay empty there. Red/green verified — a flat
single-hue redraw fails it.

Verified on the live 3080 instance: the row renders the new artwork and
"更好用的右侧边栏——文件树、编辑器、文件变动、任务和侧边对话都收在这一栏里，每个会话互不干扰。"
with no VSCode framing anywhere in the copy.

package.json

@@ -1,7 +1,7 @@
 {
   "name": "dsh-better-sidebar",
   "version": "0.21.0-alpha.1",
-  "description": "DSH web plugin: a VSCode-like right sidebar (file tree / editor / changes / tasks / side chat), isolated per conversation. Exposes a service for other plugins to register sidebar tabs and file viewers.",
+  "description": "Better Sidebar for DSH — file tree, editor, changes, tasks and side chat in one column, separate for every conversation. Also exposes a service other plugins use to register sidebar tabs and file viewers.",
   "type": "module",
   "repository": {
     "type": "git",