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/functions/server/graphics/grab-graphic-theme-vars.ts

@@ -0,0 +1,23 @@
+import fs from "fs";
+import path from "path";
+import parseThemeVars, { type ThemeVars } from "@/src/utils/parse-theme-vars";
+
+let cached: ThemeVars | null = null;
+
+/**
+ * The app's theme colours and fonts for graphics, read once from
+ * `src/styles/main.css` so a theme change there reaches them too.
+ */
+export default function grabGraphicThemeVars(): ThemeVars {
+    if (cached) return cached;
+
+    try {
+        cached = parseThemeVars({
+            css: fs.readFileSync(path.join(process.cwd(), "src/styles/main.css"), "utf-8"),
+        });
+    } catch {
+        cached = { light: {}, dark: {}, fonts: {} };
+    }
+
+    return cached;
+}