v0.1: HTTP+SSE broadcast bus + phone web UI + Linux client

Single Go module with two binaries (server, client) and an embedded
phone UI. MVP transport is HTTP POST → SSE fanout; the roadmap calls
for upgrading to WebRTC P2P with Sign in with Apple for identity, mDNS
for discovery, and OS clipboard hooks.

- server/: Go HTTP server, embedded index.html, broadcast bus with
  short replay history, SSE stream endpoint, single-binary deploy.
- client/: subscribes to SSE feed and prints messages; -send for
  one-shot publish from CLI. No OS clipboard touched yet (v0.5).
- web/index.html: dark phone-first UI, paste-clipboard button (uses
  navigator.clipboard.readText), live feed of incoming messages via
  EventSource.

This commit is intentionally tiny — it proves the end-to-end shape so
the WebRTC/SiwA/mDNS pieces can be added incrementally without
restructuring.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go.mod

@@ -0,0 +1,3 @@
+module github.com/pecord/tether
+
+go 1.22