feat(maestro): optional maestro-web systemd unit in installer (NAP-231)

After the daemon-unit step, the wizard now offers a second user unit
(`maestro-web.service`) that runs `maestro-registry` on 127.0.0.1:8888
so the web UI is reachable in a browser locally.

Decisions:

- The unit runs `maestro-registry` directly — the web bundle is already
  embedded in that binary, so a separate `maestro-web` binary would be
  pure cost. `Description=` and the unit filename make the user-facing
  framing clear.
- `Wants=maestrod.service` + `After=` (not `Requires=`): the registry
  process doesn't actually need the daemon to start; the web UI just
  has nothing to show without one. Soft dependency expresses that.
- Default loopback bind is `127.0.0.1:8888` to make it obvious which
  port is the local UI vs a separately deployed registry on `:8080`.

Wizard surface:

- New prompt (default *no*) after the daemon-systemd step.
- New flags: `--no-web`, `--web-bind <addr>`, `--web-exec-start <path>`.
- Skipped automatically when systemd is absent or `--no-systemd` is set.
- The re-run/update path mirrors the daemon flow: refresh the unit,
  daemon-reload, restart instead of start.
- `maestro-registry` is only `cargo install`ed when the user opts in,
  keeping the no-web install footprint unchanged.

docs/INSTALL.md gets the new flags + a full sample unit body.

docs/INSTALL.md

@@ -43,8 +43,13 @@ The wizard:
 3. On systemd hosts, optionally writes a user unit at
    `~/.config/systemd/user/maestrod.service`, reloads systemd, and offers to
    enable + start it.
+4. On systemd hosts, also offers a **local web UI** unit
+   (`~/.config/systemd/user/maestro-web.service`). When accepted, this also
+   `cargo install`s `maestro-registry` and runs it bound to `127.0.0.1:8888`
+   (override with `--web-bind`). Skip with `--no-web` (or just answer "n"); a
+   remote registry deployment is the alternative.
 
-Both binaries land in `~/.cargo/bin` — make sure that's on your `$PATH`.
+Binaries land in `~/.cargo/bin` — make sure that's on your `$PATH`.
 
 ## Flags
 
@@ -57,6 +62,9 @@ Both binaries land in `~/.cargo/bin` — make sure that's on your `$PATH`.
 | `--exec-start <path>` | Override the systemd unit's `ExecStart` (default `~/.cargo/bin/maestrod`). |
 | `--no-systemd` | Skip the systemd step even if detected. |
 | `--no-configure` | Skip the `maestrod configure` step. |
+| `--no-web` | Skip the optional local web-UI unit + `maestro-registry` install. |
+| `--web-bind <addr>` | Loopback bind for the web UI (default `127.0.0.1:8888`). |
+| `--web-exec-start <path>` | Override the web unit's `ExecStart` (default `~/.cargo/bin/maestro-registry`). |
 | `-y`, `--yes` | Non-interactive: install binaries + drop systemd unit + reload + enable, but **don't** auto-run `configure` or auto-start the unit (those need real config). |
 
 ## What the systemd unit looks like
@@ -87,6 +95,31 @@ systemctl --user status maestrod
 journalctl --user -u maestrod -f
 ```
 
+The optional `maestro-web.service` looks like:
+
+```ini
+[Unit]
+Description=Maestro local web UI (maestro-registry on 127.0.0.1:8888)
+Documentation=https://git.naps.pt/naps62/maestro
+After=network-online.target maestrod.service
+Wants=maestrod.service
+
+[Service]
+Type=simple
+Environment=MAESTRO_REGISTRY_BIND_ADDR=127.0.0.1:8888
+ExecStart=%h/.cargo/bin/maestro-registry
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=default.target
+```
+
+`Wants=maestrod.service` (not `Requires=`) — the registry process is
+technically independent of the daemon, but the web UI has nothing to show
+without one connected, so we order this after `maestrod` and politely
+encourage it.
+
 ## Running the wizard without the bootstrap script
 
 If you'd rather skip the script: