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/export-message-text.test.ts

@@ -20,6 +20,10 @@ describe("exportMessageText", () => {
         );
     });
 
+    test("words graphics by number", () => {
+        expect(exportMessageText({ text: "See [graphic:5]", ...names, heading_offset: 0 })).toBe("See (graphic 5)");
+    });
+
     test("leaves refs that are already links alone", () => {
         expect(exportMessageText({ text: "[bot:3](#x)", ...names, heading_offset: 0 })).toBe("[bot:3](#x)");
     });