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/data/models/TikzCache.ts

@@ -0,0 +1,6 @@
+import { DataType } from "@/data/DataType.enum"
+import { Model } from "@/data/models/Model"
+
+export interface TikzCache extends Model<DataType.TikzCache> {
+  svg: string
+}