README: page address, failed-check band, timestamp form, YAML keys (closes #210)

Getting Started names the address to open for `make dev` and the image,
and says `make run` serves no page. Rows, colors and controls says a
failed check is a faint red band in the graph. Reporting gives the
`timestamp` form. Settings gives an example line of the YAML file.

Model: opus-5-5

README.md

@@ -22,7 +22,7 @@ make setup
 # Serve the page from the Vite dev server
 make dev
 
-# Build the page and netwatch-server, then run the server on port 8080
+# Build the page and netwatch-server, then run only the server, on port 8080
 make run
 
 # Run the tests, both linters and the format check
@@ -36,9 +36,16 @@ make docker
 docker run -p 8080:8080 netwatch
 ```
 
-`make check` and `make docker` need Docker. `make run` starts `netwatch-server`
-on port 8080 with its defaults, so the page's reports are stored in
-`data/reports`. `make dev` passes the page's `/api` requests there.
+After `make dev`, open the page at `http://localhost:5173/`. From the image, it
+is at `http://localhost:8080/`.
+
+`make run` serves no page: `http://localhost:8080/` answers
+`404 page not found`. It starts `netwatch-server` on port 8080 with its
+defaults, which stores reports in `data/reports`. `make dev` passes the page's
+`/api` requests there, so run `make run` in a second terminal to store the
+page's reports.
+
+`make check` and `make docker` need Docker.
 
 ## The page
 
@@ -113,9 +120,9 @@ half second. As soon as one answers, a new round starts.
 ### Rows, colors and controls
 
 A figure and its line in the graph take the color of the legend in the footer,
-from green under 50 ms to red from 500 ms up; gray is no answer. The graph's
-axis ends at 1000 ms; a higher latency is drawn at the top, and the figure still
-shows its real value.
+from green under 50 ms to red from 500 ms up. A failed check has a gray figure,
+and a faint red band in the graph. The graph's axis ends at 1000 ms; a higher
+latency is drawn at the top, and the figure still shows its real value.
 
 From time to time, the page sorts the WAN rows by their latest latency: fastest
 first, failed last. The pin button at the left of a row keeps the row at the
@@ -140,7 +147,9 @@ fields:
   and `history`, its samples. A sample has the check's time `t` in Unix
   milliseconds, its `latency` (`null` for a failed check) and its `error`
   (`"timeout"`, `"unreachable"` or `null`).
-- `timestamp`: the time of the report
+- `timestamp`: the time of the report, as an ISO 8601 string such as
+  `2026-11-11T20:01:35.685Z`. A report whose `timestamp` is a number is refused
+  with 400.
 
 Each report carries only the samples no earlier report has delivered. A sample
 waits as long as the history holds it: the last 100 rounds of each host. An
@@ -218,8 +227,9 @@ reason is in its log.
 `netwatch-server` reads its settings from environment variables. It also reads a
 `.env` file in the directory it starts in, and a YAML file named
 `netwatch-server.yaml` in `/etc/netwatch-server/` or
-`~/.config/netwatch-server/`, when one exists. A variable in the environment
-wins over a file.
+`~/.config/netwatch-server/`, when one exists. In the YAML file, each key is a
+variable's name, in upper or lower case: `PORT: 8090` sets the port. A variable
+in the environment wins over a file.
 
 The server does not start when a variable holds a value it cannot use. Its error
 names the variable. An empty variable counts as unset, except `TRUSTED_PROXIES`.