Render markdown preview in-browser (WASM)

#10 in tonybierman/dx-blog — merged 2026-05-29

What

Moves the post editor's live Markdown preview off the server and into the wasm client. Implements the In-browser WASM markdown editor roadmap item.

Previously the editor round-tripped to the preview_markdown server fn on every (debounced) keystroke. Now render_markdown (the shared pulldown-cmark + ammonia pipeline) is compiled for the web target and runs directly in the browser.

Why it's a good fit

The client now uses the exact same pipeline the server uses to produce stored body_html, so the preview matches saved output byte-for-byte. It's also:

• Zero-latency — synchronous use_memo, no debounce and no network round-trip
• Offline-capable — preview works without the server
• Zero server load — the preview endpoint is gone entirely

Changes

• Cargo.toml — add pulldown-cmark + ammonia to the web feature (verified they compile to wasm32-unknown-unknown)
• src/server/mod.rs — gate render_markdown on any(server, web)
• src/pages/admin/posts.rs — replace the server-fn use_resource with a synchronous use_memo; drop the preview debounce
• src/server/admin/posts.rs — delete the now-unused preview_markdown server fn (POST /api/admin/preview)

Verification

• Server check (server,sqlite): clean
• WASM client check (web, wasm32-unknown-unknown): clean
• cargo fmt: applied
• Tests: 11/11 pass — including render_markdown sanitization tests, which now also cover the in-browser path

Notes / follow-ups

• Adds ammonia/html5ever to the wasm bundle (size trade-off), kept identical to the server pipeline so preview stays byte-for-byte consistent.
• Syntax highlighting (syntect/tree-sitter) in code blocks is a noted follow-up, not in this PR.

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