Owner-only admin page: Reader roster plus Poll Lane status (#102) (#107)

Closes #102.

The only operational surface was /healthz and a fold-out roster inside the owner's own reading page. This adds /admin: an owner-only page carrying the Reader roster and one row per Poll Lane.

- **Poller seam.** `latest.Poller` records each Lane's last pass (`Site`, `Due`, `Checked`, `LastRun`, `Gap`, `Clamped`, `Browser`) and answers `LaneStatus()`; the page reads that snapshot, never a table. A pass that returns before computing its figures (refusal backoff, sidecar down) carries the previous pass's figures forward rather than recording zeroes, and a Lane that has never reached a pace renders no gap at all. Refusal and sidecar reachability are derived at snapshot time.
- **Owner gate at registration.** Every route reaching past the acting Reader lives in `adminRoutes()` and is wrapped in `requireOwner` when it is registered, so a missing gate is visible in the route list rather than hidden in a handler. `web.AdminPatterns()` is what the gate test walks, so a new route cannot be added without being tested. A non-owner gets 404, never 403.
- **Nil poller is a first-class state.** `main.newRouter` takes the reporter as an interface and converts a nil `*Poller` to a nil interface; no poller and no completed pass both render "No data yet" with the reason spelled out, rather than confident zeroes.
- **Roster moved** off the reading page onto /admin, with the Sighting counters and a confirm-gated `Clear marks` control. #103 fills those counters, so on delivery they read zero for everyone - deliberate ordering.
- **One accent, `--patina`** (verdigris, both colour branches): the far side of the wheel from ember's crimson and clear of the archive blue. Ember still means new chapter only; revocation still wears --danger.

Verification: `go vet ./...` and `go test ./...` green (Docker-backed); admin page screenshotted at 1100px and 390px in both colour schemes. Reviewed on both axes (spec, standards); findings on the accent hue, zero-figure honesty and three tests that could not fail are fixed in 58014eb.
Reviewed-on: #107
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/reader_credential_test.go

@@ -49,7 +49,7 @@ func withBody(req *http.Request, body string) *http.Request {
 // A refused credential is refused however plausible it looks: only a hash the
 // readers table holds authenticates anything.
 func TestUnknownCredentialRejected(t *testing.T) {
-	srv := newRouter(newTestStore(t), testConfig())
+	srv := newRouter(newTestStore(t), testConfig(), nil)
 
 	rr := httptest.NewRecorder()
 	srv.ServeHTTP(rr, credRequest(http.MethodGet, "/bookmarks", readerCredential("never-registered")))
@@ -69,7 +69,7 @@ func TestUnknownCredentialRejected(t *testing.T) {
 func TestPerReaderIsolation(t *testing.T) {
 	s := newTestStore(t)
 	registerReader(t, s, "other-reader")
-	srv := newRouter(s, testConfig())
+	srv := newRouter(s, testConfig(), nil)
 
 	ownerKey := "asura:solo"
 	putBookmark(t, srv, ownerKey, store.Bookmark{
@@ -267,7 +267,7 @@ func TestRotateCredentialViaWebUI(t *testing.T) {
 	}
 	cfg := testConfig()
 	cfg.UserscriptPath = path
-	srv := newRouter(s, cfg)
+	srv := newRouter(s, cfg, nil)
 
 	oldCred := ownerCredential()
 	rr := httptest.NewRecorder()