docs+release: Phase 4 launch artifacts

README hero with real screenshots, install/config/widget docs, goreleaser
for dash + dash-agent, GHCR release workflow, changelog, community files,
agent systemd unit and Dockerfile, version stamping via -X main.version.

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docs/installation.md

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+# Installation
+
+Dash ships as a single container image or a single static binary. All state
+lives in one directory: a SQLite database and uploaded icons.
+
+## Docker (recommended)
+
+```sh
+docker run -d --name dash \
+  -v ./data:/data \
+  -p 3000:3000 \
+  --restart unless-stopped \
+  ghcr.io/dvorinka/dash:latest
+```
+
+Or compose:
+
+```yaml
+services:
+  dash:
+    image: ghcr.io/dvorinka/dash:latest
+    ports:
+      - "3000:3000"
+    volumes:
+      - ./data:/data
+    restart: unless-stopped
+```
+
+Open http://localhost:3000.
+
+To build the image locally instead of pulling:
+
+```sh
+docker compose up --build
+```
+
+## Binary
+
+Release archives contain a static `dash` binary (no CGO, no dependencies).
+
+```sh
+tar xzf dash_linux_amd64.tar.gz
+./dash   # listens on :8080, stores data in ./data
+```
+
+Set `DASH_PORT` and `DASH_DATA_DIR` to change the defaults.
+
+## dash-agent (system monitoring)
+
+The agent collects CPU, memory, disk, network, load, temperatures, and Docker
+container state, then pushes a JSON sample to Dash every few seconds.
+
+1. In the UI, go to **Systems → New system**, copy the generated token.
+2. Download the `dash-agent` binary for the host's architecture from the
+   release page, or build it:
+
+   ```sh
+   cd apps/backend && go build -o dash-agent ./cmd/dash-agent
+   ```
+
+3. Run it:
+
+   ```sh
+   dash-agent -url http://dash:3000 -token <token>
+   ```
+
+### Flags / environment
+
+| Flag | Env | Default | Purpose |
+|---|---|---|---|
+| `-url` | `DASH_URL` | — | Dash base URL |
+| `-token` | `DASH_TOKEN` | — | system token from the UI |
+| `-interval` | `DASH_INTERVAL` | `10` | push interval, seconds |
+| `-once` | — | `false` | print one sample and exit (no push) |
+
+If `/var/run/docker.sock` exists, container states are included automatically.
+
+### systemd
+
+`packaging/dash-agent.service` is a ready-made unit:
+
+```sh
+sudo install -m755 dash-agent /usr/local/bin/dash-agent
+sudo install -m644 packaging/dash-agent.service /etc/systemd/system/
+sudo mkdir -p /etc/dash
+printf 'DASH_URL=http://dash:3000\nDASH_TOKEN=<token>\n' | sudo tee /etc/dash/agent.env
+sudo systemctl enable --now dash-agent
+```
+
+### Docker (limited)
+
+`infra/Dockerfile.agent` builds a minimal agent image. A containerized agent
+only sees its own `/proc` — mount the host's procfs and use host PID mode for
+meaningful numbers:
+
+```sh
+docker run -d --name dash-agent --pid=host \
+  -v /proc:/host/proc:ro \
+  -v /var/run/docker.sock:/var/run/docker.sock:ro \
+  -e DASH_URL=http://dash:3000 -e DASH_TOKEN=<token> \
+  ghcr.io/dvorinka/dash-agent:latest
+```
+
+The systemd install above is the recommended path.
+
+## Reverse proxy
+
+Dash serves the UI and API on one port — a plain reverse proxy works:
+
+```nginx
+location / {
+    proxy_pass http://127.0.0.1:3000;
+    proxy_set_header Host $host;
+}
+```
+
+## Upgrading
+
+Pull the new image and restart, or replace the binary. Goose migrations run
+automatically on boot; `data/` carries forward.
+
+```sh
+docker pull ghcr.io/dvorinka/dash:latest && docker compose up -d
+```