Target check timeout is 80% of the refresh interval (closes #78)

Each target check now times out after 80% of the refresh interval, 24
seconds at 30 seconds, where it was capped at 3 seconds, so slow, far
targets are recorded with their real time. A round asked for while the
last one's checks are still waiting, after an interval change or by the
recovery probe, is skipped, so rounds never overlap; the recovery probe
starts no new checks while its last ones wait.

The frontend has its first unit tests, run by script/frontend-test with
Node's built-in test runner. index.html now links src/styles.css, which
src/main.js imported, since Node cannot import CSS.

Model: opus-5-5

README.md

@@ -52,8 +52,8 @@ halves, so the root `make check` fails if either one is broken. We provide:
 - `script/fmt` — format all files (writes): prettier, then gofmt over `backend/`
 - `script/fmt-check` — check formatting (read-only): prettier, then gofmt
 - `script/check` — run test, lint, and fmt-check
-- `script/frontend-test` — run the production build as the frontend's test (no
-  unit tests yet)
+- `script/frontend-test` — run the unit tests in `test/unit/` with Node's
+  built-in test runner, then the production build
 - `script/frontend-lint` — run prettier in check mode
 - `script/frontend-fmt` — format everything prettier understands (writes)
 - `script/frontend-fmt-check` — check prettier formatting (read-only)
@@ -136,8 +136,11 @@ Local hosts are tracked separately from WAN stats.
 ### Latency measurement
 
 HEAD requests with `mode: 'no-cors'` and `cache: 'no-store'`, timed with
-`performance.now()`. 1-second timeout; anything over 1000ms is clamped to
-unreachable. IPv4 only.
+`performance.now()`. Each check times out after 80% of the refresh interval (24
+seconds at 30 seconds) and is then recorded as a timeout, so a round's checks
+have all finished before the next round is due. A round due while the last one
+is still waiting, which happens only after an interval change or when the
+recovery probe starts one, is skipped. IPv4 only.
 
 ### Color coding