Add bot graphics

Bots can now show a concept as a visual: create_graphic (with
update_graphic, read_graphic and list_graphics, on every run, no computer
needed) stores one HTML document in bot_graphics, per user like the
conversation, and the chat renders its [graphic:ID] ref as an inline card
sized to fit, with a large view and a page of its own (no rail).

Graphic HTML is treated as hostile: it is only served at /graphics/<id>
with a CSP sandbox allowing scripts under an origin of its own and no
network, loading only the vendored Chart.js and Mermaid from
/graphics-lib/. The document carries the app's theme vars and scrollbars,
reports its height to the thread, and a frame that navigates away is
closed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/utils/graphic-ref.ts

@@ -0,0 +1,28 @@
+type Params = {
+    id: number | string;
+};
+
+/**
+ * How a bot names a graphic it made: `[graphic:5]`. The chat turns it
+ * into a link (`linkComputerRefs`) that renders as the graphic itself,
+ * framed in the thread, with ways to open it full size.
+ */
+export default function graphicRef({ id }: Params) {
+    return `[graphic:${id}]`;
+}
+
+/**
+ * The link target a graphic ref becomes, e.g. `#graphic-5`.
+ */
+export const GraphicRefHrefPrefix = "#graphic-";
+
+/**
+ * The id in a graphic ref's link target, or null.
+ */
+export function graphicRefId({ href }: { href?: string | null }) {
+    if (!href?.startsWith(GraphicRefHrefPrefix)) return null;
+
+    const id = Number(href.slice(GraphicRefHrefPrefix.length));
+
+    return Number.isInteger(id) && id > 0 ? id : null;
+}