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 cannot be created or
read stopped the start with an error that named the path but not
DATA_DIR. Both errors now read DATA_DIR "<value>": <reason>, the form
config.New uses for the other settings. A test starts the buffer with
DATA_DIR below a regular file and checks the error.

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 +21,15 @@ 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 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)); it named only the
+  path. A unit test starts the buffer with `DATA_DIR` below a regular file and
+  checks 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: a browser that cannot format the header's clocks still probes
   ([#149](https://git.eeqj.de/sneak/netwatch/issues/149)). The clocks name the
   local time zone through `Intl.DateTimeFormat`'s `shortGeneric` option, which