[local-explorer-ui] Add email view and sending emails to workers to the local explorer (#15337)

Author: tpmmorris

.changeset/email-local-explorer-ui.md

@@ -0,0 +1,10 @@
+---
+"@cloudflare/local-explorer-ui": minor
+"miniflare": minor
+---
+
+Add email inspection and testing to Local Explorer
+
+Add an Email group with Routing and Sending views for inspecting messages received by a Worker's `email()` handler and messages sent through its `send_email` bindings. Detail views show message content, metadata, attachments, and handler activity including forwarding, replies, rejection, and unhandled messages.
+
+Add a test-email composer that delivers custom text, HTML, headers, and attachments directly to the selected Worker's `email()` handler during local development.

.changeset/validate-test-email-headers.md

@@ -0,0 +1,7 @@
+---
+"miniflare": patch
+---
+
+Validate custom headers sent through the Local Explorer test-email endpoint
+
+Reject header names and values that cannot be safely encoded. Multiline values remain supported and are folded into valid MIME continuation lines.

packages/local-explorer-ui/package.json

@@ -47,6 +47,7 @@
 	},
 	"devDependencies": {
 		"@cloudflare/workers-tsconfig": "workspace:*",
+		"@cloudflare/workers-utils": "workspace:*",
 		"@hey-api/openapi-ts": "catalog:default",
 		"@tanstack/react-router-devtools": "^1.158.0",
 		"@tanstack/router-plugin": "^1.158.0",