feat(frontend): post collected samples to /api/v1/reports (closes #53)

A Reporter class beside AppState POSTs a JSON report to the same-origin
/api/v1/reports every reportInterval (default 60s, a new CONFIG value).
buildReport is a pure function of host state: it emits each host's
unreported, non-paused samples ({t, latency, error}) plus a random
per-browser clientId from localStorage, geo null, and an ISO 8601 UTC
timestamp. A per-host high-water mark keyed on sample time makes every
report a delta and only advances on a delivered POST, so a failed send
re-sends next interval, bounded by the existing history window. Failure
is quiet: one debug-log line per outage, one on recovery, no alert, no
tight retry loop, never blocking probing. While paused nothing is sent.

vite.config.js proxies /api to 127.0.0.1:8080 so yarn dev exercises the
real path against a local netwatch-server. Backend and schema unchanged.

Model: opus-4-8

README.md

@@ -23,6 +23,9 @@ docker build -t netwatch .
 docker run -p 8080:8080 netwatch
 ```
 
+`yarn dev` proxies `/api` to `http://127.0.0.1:8080`, so a locally running
+`netwatch-server` (see `backend/`) receives the reports the page posts.
+
 ## Entrypoints
 
 This repository adheres to the
@@ -71,6 +74,18 @@ code lives in `src/main.js` with a class-based architecture:
 - **`tick()`**: Main loop — measures all hosts in parallel via `Promise.all`,
   pushes samples, redraws UI. When paused, pushes blank markers (no probes, no
   false outage)
+- **`Reporter`**: Posts collected samples to the backend
+
+### Reporting
+
+Every `reportInterval` (default 60s) the page POSTs a JSON report to the
+same-origin path `/api/v1/reports`: a random per-browser `clientId` kept in
+`localStorage`, `geo` sent as null, and each host's unreported, non-paused
+samples (timestamp, latency, error). A per-host high-water mark makes every
+report a delta, so only new samples are sent; the mark advances only on a
+delivered report, and while paused nothing is sent. Delivery failure is quiet —
+one debug-log line per outage, retried at the next interval, never blocking
+probing. The report-building step is a pure function of host state.
 
 ### Monitoring targets