Signal chats in the app (desktop only) with a chat button on every screen

- signal.js: talks to signal-cli-rest-api (a linked device of your Signal account); keeps conversations in /data/signal; live updates; send/receive text, files, replies, reactions, groups, receipts
- Signal tab and a floating chat panel on every screen, shown only on desktop-class windows; new Signal permission per person
- compose: signal bridge container sharing the hub's network
- tools/signal-test.js (fake bridge), README

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Dockerfile

@@ -2,7 +2,7 @@ FROM node:22-alpine
 WORKDIR /app
 COPY package.json package-lock.json ./
 RUN npm ci --omit=dev
-COPY server.js auth.js cal.js ics.js caldav.js tasks.js settings.js search.js users.js admin.js vault.js ./
+COPY server.js auth.js cal.js ics.js caldav.js tasks.js settings.js search.js users.js admin.js vault.js signal.js ./
 COPY public ./public
 ENV PORT=8080 DATA_DIR=/data
 RUN mkdir -p /data && chown node:node /data