refactor: split admin.ts into sub-modules (P2-11)

Extracts 3833-line admin.ts into focused modules:
- src/routes/admin/ui.ts       — layout() and clampText() helpers
- src/routes/admin/helpers.ts  — KV list helpers (listAllFeeds, addFeedToList, etc.)
- src/routes/admin/feeds.ts    — feed CRUD routes (feedsRouter)
- src/routes/admin/emails.ts   — email view/delete routes (emailsRouter)

admin.ts now mounts the sub-routers and retains only auth middleware,
dashboard, login/logout, and the in-place feed API update route.
All 163 tests continue to pass.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/routes/admin/ui.ts

@@ -0,0 +1,36 @@
+import { html, raw } from "hono/html";
+import { designSystem } from "../../styles/index";
+import { interactiveScripts } from "../../scripts/index";
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export const layout = (title: string, content: any) => {
+  return html`<!DOCTYPE html>
+    <html>
+      <head>
+        <title>${title} - Email to RSS Admin</title>
+        <meta charset="UTF-8" />
+        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+        <meta name="color-scheme" content="light dark" />
+        <style>
+          ${raw(designSystem)}
+        </style>
+        <script>
+          ${raw(interactiveScripts)};
+        </script>
+      </head>
+      <body class="page">
+        ${content}
+      </body>
+    </html>`;
+};
+
+export function clampText(value: string, maxLen: number): string {
+  const raw = `${value || ""}`;
+  if (raw.length <= maxLen) {
+    return raw.trim();
+  }
+  if (maxLen <= 3) {
+    return raw.slice(0, maxLen).trim();
+  }
+  return `${raw.slice(0, maxLen - 3).trimEnd()}...`;
+}