feat: browser-only screen mirroring prototype

A FastAPI server with two pages and one WebSocket. /mirror captures a
window, screen or tab through the OS-native getDisplayMedia picker; /watch
shows every live mirror as a wall and takes one fullscreen on click. Any
number of sharers and watchers, at the same time.

Media never touches the server: each watcher gets its own RTCPeerConnection
to the sharer, and the Python process only relays JSON. The roster lives in
registry.py, which is transport-free by design -- every operation returns
the envelopes to deliver, so admission, fan-out and teardown policy are all
unit-testable without a socket.

Registry rules worth stating: a signal frame is relayed only along an
existing subscription in either direction; viewer-to-peer traffic is
refused; and teardown is broadcast to every viewer rather than only the
subscribers, because a tile is drawn on stream_added and a viewer that has
not yet subscribed still has a stale tile to clear.

Frontend is hand-written ES modules with no build step and no external
asset. Chromium scales a monitor capture to the capturing window and
requires a secure context, so /mirror detects an insecure origin and prints
the three ways out rather than failing on a button click.

LLM-Generated: true

LICENSE

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+MIT License
+
+Copyright (c) 2026 mirror contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.