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.

package.json

@@ -37,6 +37,7 @@
     "arktype": "^2.1.29",
     "comlink": "^4.4.2",
     "date-fns": "^4.1.0",
+    "dompurify": "^3.4.3",
     "events": "^3.3.0",
     "font-color-contrast": "^11.1.0",
     "fontfaceobserver": "^2.3.0",