feat: render Markdown, mathematics and Mermaid diagrams server-side

internal/diagram/testdata/fc-shapes.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1256 152" font-family="sans-serif"><rect x="56" y="54" width="96" height="44" fill="#ffffff" stroke="#333" stroke-width="1.5"/><text x="104" y="81" text-anchor="middle" font-size="14">Rect</text><rect x="176" y="54" width="96" height="44" rx="10" fill="#ffffff" stroke="#333" stroke-width="1.5"/><text x="224" y="81" text-anchor="middle" font-size="14">Round</text><polygon points="380,34 464,76 380,118 296,76" fill="#ffffff" stroke="#333" stroke-width="1.5"/><text x="380" y="81" text-anchor="middle" font-size="14">Diamond</text><rect x="488" y="32" width="88" height="88" rx="44" fill="#ffffff" stroke="#333" stroke-width="1.5"/><text x="532" y="81" text-anchor="middle" font-size="14">Circle</text><rect x="600" y="54" width="108" height="44" fill="#ffffff" stroke="#333" stroke-width="1.5"/><line x1="605" y1="76" x2="703" y2="76" stroke="#333" stroke-width="1.5"/><text x="654" y="81" text-anchor="middle" font-size="14">Subroutine</text><polygon points="732,54 822,54 840,76 822,98 732,98" fill="#ffffff" stroke="#333" stroke-width="1.5"/><text x="786" y="81" text-anchor="middle" font-size="14">Asymmetric</text><rect x="864" y="54" width="96" height="44" rx="22" fill="#ffffff" stroke="#333" stroke-width="1.5"/><text x="912" y="81" text-anchor="middle" font-size="14">Stadium</text><polygon points="1004,54 1060,54 1080,76 1060,98 1004,98 984,76" fill="#ffffff" stroke="#333" stroke-width="1.5"/><text x="1032" y="81" text-anchor="middle" font-size="14">Hexagon</text></svg>