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/internal/web/templates/admin.html

@@ -0,0 +1,39 @@
+{{/* The owner's administrative page: everything that reaches past one Reader,
+     at its own address so it can be bookmarked rather than hunted for inside
+     the reading page. Owner-only at route registration (requireOwner), which
+     is why nothing in here re-tests who is asking. */}}
+{{define "admin"}}
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
+  <meta name="color-scheme" content="dark light">
+  <title>BookmarkManager — Admin</title>
+  <link rel="icon" href="/static/logo.svg" type="image/svg+xml">
+  <link rel="stylesheet" href="/static/style.css">
+  <link rel="preload" href="/static/fonts/instrument-serif-400-latin.woff2" as="font" type="font/woff2" crossorigin>
+  <script src="/static/htmx.min.js" defer></script>
+</head>
+<body>
+<div class="sheet">
+  <header class="topbar">
+    <h1 class="brand">{{template "mark" .}}<span>Bookmark<em>Manager</em></span></h1>
+    {{/* Back to the library, no switch: this page belongs to neither library,
+         and the ember-lit switch says which library you are reading. */}}
+    <a class="ghost back" href="/">Library</a>
+    <form method="post" action="/logout">
+      <button type="submit" class="ghost">Log out</button>
+    </form>
+  </header>
+
+  {{/* The live region wraps the swapped block rather than being it: the
+       refresh replaces the section wholesale, and a region recreated on every
+       update is never announced. */}}
+  <div aria-live="polite">{{template "lanes" .Lanes}}</div>
+
+  {{template "readers" .}}
+</div>
+</body>
+</html>
+{{end}}