feat(markdown): render TikZ fenced blocks to SVG

Adds a markdown-it extractor for ```tikz blocks that emits a sanitized
placeholder, plus a runTikz() that lazy-loads the obsidian-tikzjax 0.5.2
bundle from jsdelivr on first encounter, sanitizes the produced SVG with
DOMPurify, and caches it in PouchDB keyed by sha256(source) so repeat
views are instant and offline-capable.

src/components/StackedNote.vue

@@ -16,6 +16,7 @@ import { useLinks } from "@/hooks/useLinks.hook"
 import {
   renderCodeFile,
   runMermaid,
+  runTikz,
   useShikiji
 } from "@/hooks/useMarkdown.hook"
 import { useNoteFreshness } from "@/hooks/useNoteFreshness.hook"
@@ -160,6 +161,10 @@ watch([content, mode], () => {
       runMermaid(`.note-${sha.value} .mermaid`)
     }
 
+    if (rawContent.value.includes("```tikz")) {
+      void runTikz(`.note-${sha.value} .tikz`)
+    }
+
     if (isMarkdown.value && rawContent.value.includes("```")) {
       useShikiji()
     }