Unusable DATA_DIR error names DATA_DIR; report file number is atomic.Int64 (closes #148, closes #161)

When the backend runs on its own, a DATA_DIR that could not be created
or read stopped the start with an error that named the path but not
DATA_DIR, and one where no file could be created let it start, to fail
at the first write of a report file. The start now creates a file in
DATA_DIR and removes it, and all three cases fail with
DATA_DIR "<value>": <reason>, the form config.New uses for the other
settings. Tests start the buffer with DATA_DIR below a regular file and
with DATA_DIR set to /proc.

The report file sequence counter is atomic.Int64 instead of
atomic.Uint64, as the Go style guide asks for int types. File names are
unchanged.

Model: opus-5-5

TODO.md

@@ -21,6 +21,17 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-18: when the backend runs on its own, a `DATA_DIR` that cannot be
+  created or read, or where no file can be created, stops the start with an
+  error naming `DATA_DIR` and its value, as the other settings' errors do
+  ([#148](https://git.eeqj.de/sneak/netwatch/issues/148)); the first two named
+  only the path, and the third was found only at the first write of a report
+  file. At start the server creates a file in `DATA_DIR` and removes it. Unit
+  tests start the buffer with `DATA_DIR` below a regular file and with
+  `DATA_DIR` set to `/proc`, and check the error. The report file number is an
+  `atomic.Int64`, as the Go style guide asks for int types
+  ([#161](https://git.eeqj.de/sneak/netwatch/issues/161)); file names are
+  unchanged
 - 2026-10-18: the image's nginx sends the page with `Cache-Control: no-cache`
   ([#141](https://git.eeqj.de/sneak/netwatch/issues/141)), so a browser that
   keeps a copy checks with the server that it is current before using it again.