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

A Reporter beside AppState POSTs a JSON delta report of each host's
unreported, non-paused samples to the same-origin /api/v1/reports every
reportInterval (default 60s). buildReport is an exported pure function of
host state; init() runs only when #app exists, so a test can import it.
Per-host marks advance only on a delivered POST and never move backwards;
only one POST is in flight, abandoned after half the interval, so a slow
backend cannot cause re-sent samples. Failure is quiet and never blocks
probing.

The client id falls back to crypto.getRandomValues where crypto.randomUUID
is missing (plain HTTP to a non-localhost host). vite.config.js proxies
/api to 127.0.0.1:8080 for yarn dev.

Model: opus-5-5

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
@@ -85,6 +88,18 @@ 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