docs: update all docs with dev server, live reload, Juice inlining

- README.md: restructure Preview section into Static + Dev modes
  with feature comparison table; mention juice inlining and live reload
- CONTRIBUTING.md: update Development Setup with Go/Node.js
  requirements, static vs dev preview workflows, Juice reference
- AGENTS.md: add dev command to subcommand list, document
  tools/server/ directory, update Build Tool section
- docs/CONTRIBUTING.*.md (5 languages): add dev server section,
  Node.js requirement, live reload description
- docs/README.*.md (5 languages): add dev mode to Preview section
- docs/images/README.md: update capture instructions for dev server

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/images/README.md

@@ -11,8 +11,9 @@ neon.png       mono.png        terra.png    ink.png      aurora.png
 
 ## How to Capture
 
-1. Run `cd tools && go run . preview all` from the project root
-2. Open `preview/index.html` in a browser
-3. For each style, select the "Activate Account" template and "Modern" client mode
-4. Take a screenshot of the rendered email (600px width recommended)
-5. Save as `<style-name>.png` in this directory
+1. Start the dev server: `cd tools && go run . dev` (opens http://localhost:3456)
+2. For each style, select the "Activate Account" template and "Modern" client mode
+3. Take a screenshot of the rendered email (600px width recommended)
+4. Save as `<style-name>.png` in this directory
+
+> For static preview, run `cd tools && go run . preview all` then open `preview/index.html` directly.