docs: add spec for removing juice and simplifying preview module

docs/superpowers/specs/2026-06-23-remove-juice-simplify-preview-design.md

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+# Spec: Remove Juice & Simplify Preview Module
+
+**Date:** 2026-06-23
+**Status:** approved
+
+## Goal
+
+Remove multi-email-client simulation (Gmail/Outlook) from the preview module, eliminate the `juice` CSS-inlining dependency, and rewrite the dev server in pure Go with SSE-based live reload.
+
+## Motivation
+
+- Multi-client CSS simulation has diminishing value — modern email clients render consistently
+- `juice` + Node.js adds complexity (npm install, node_modules, separate runtime)
+- The project already uses Go for all build tooling; a Node.js dev server is an outlier
+- Simplifying to "Modern" and "Source" views covers the real use cases
+
+## Changes
+
+### 1. Delete `tools/server/` (entire directory)
+
+Remove all Node.js artifacts:
+- `inliner.mjs` — juice CSS inlining + Gmail/Outlook CSS stripping
+- `server.mjs` — Express + WebSocket dev server
+- `package.json` / `package-lock.json`
+- `node_modules/` — all npm dependencies
+
+### 2. Rewrite `tools/cli/dev.go`
+
+- Remove Node.js dependency check
+- Remove `exec.Command("node", "server.mjs")` subprocess
+- Instead: create and start a pure Go HTTP server (calling into `tools/preview/server.go`)
+- Keep the same CLI interface: `go run . dev [--port <port>]`
+
+### 3. New file: `tools/preview/server.go`
+
+Pure Go dev server with:
+- **Static file serving** — serve `preview/` directory (index.html, rendered.js)
+- **SSE endpoint** (`GET /events`) — Server-Sent Events for browser reload notifications
+- **File watcher** — poll `themes/` every 500ms, compare mod times of `.tmpl` files
+- **On change detected** — call `preview.RenderAll()` directly (in-process, no subprocess), write `rendered.js`, broadcast SSE `reload` event
+- No external Go dependencies — uses `net/http` stdlib only
+
+### 4. Modify `preview/index.html`
+
+Frontend changes:
+- **Client selector** — reduce from 4 options (Modern/Gmail/Outlook/Raw Source) to 2 (Modern/Source)
+- **Remove** `renderedGmail` / `renderedOutlook` variables and all Gmail/Outlook JS logic
+- **Simplify** `getRendered()` — always returns `rendered`
+- **Simplify** `transform()` — only handles `source` mode (HTML escaping)
+- **Remove** Client Simulation indicators panel (HTML section + JS in `updatePanel()`)
+- **Remove** static-mode warning message ("Gmail / Outlook simulation is approximate...")
+- **Replace** WebSocket with SSE (`new EventSource('/events')`)
+- **SSE reload handler** — dynamically reload `rendered.js` on `reload` event, re-render iframe without losing current theme/template/viewport selection
+- **Simplify** `setDevMode()` — remove multi-client disclaimer
+
+### 5. Update `AGENTS.md`
+
+- Remove references to Juice, Node.js, Gmail/Outlook variants
+- Update dev server description to reflect pure Go implementation
+
+## Non-Changes
+
+- `tools/preview/engine.go` — template rendering engine unchanged
+- `tools/preview/funcs.go` — template functions unchanged
+- `tools/preview/locale.go` — locale data unchanged
+- `tools/config/` — config loading unchanged
+- `tools/data/templates_config.json` — unchanged
+- `tools/cli/preview.go` — preview command unchanged
+- `tools/cli/commands.go` — command registration unchanged
+- All theme `.tmpl` files — unchanged
+
+## Behavior
+
+### Static preview (open `preview/index.html` directly)
+- Works via `file://` protocol as before
+- Two view modes: Modern (rendered HTML in iframe) and Source (escaped HTML source)
+- No dev-mode warnings needed
+
+### Dev mode (`go run . dev`)
+- `go run . dev` starts the Go HTTP server on port 3456 (configurable)
+- On startup: runs preview engine once, writes `rendered.js`
+- Serves `preview/` as static files
+- Watches `themes/` for `.tmpl` changes (500ms polling)
+- On change: re-renders affected themes, updates `rendered.js`, pushes SSE event
+- Browser auto-reloads preview content without page refresh (preserves UI state)
+
+## Constraints
+
+- No new Go dependencies — SSE and file polling use stdlib only
+- No Node.js requirement
+- `preview/rendered.js` format stays compatible: `window.__RENDERED__`, `window.__REGISTRY__`, `window.__PARAMS__`
+- Preview still works with `file://` protocol (no server needed for basic use)