Optional Senpai integration: "Who's here" side sheet and /api/senpai proxy

- server.js: GET /api/senpai/v1/{locate,xray,characters/<ref>[/timeline],portraits/<ref>} forwarded to
  SENPAI_URL (plain http/https, no dependencies) with SENPAI_TOKEN, X-Senpai-User and the user's horizon
  (furthest chapter with progress, its version and furthest page); off when SENPAI_URL is unset
- progress records "top", the furthest page read in a version, so paging back does not lower the horizon
- /api/me reports senpai: true|false
- reader: Who's here button (key x) with This page / This chapter / So far, character card, timeline with
  jump links, figure outlines on the page; options "only up to this page" and "show unconfirmed"
- read.html?...&p=<index> opens a page; readUrl takes p

public/common.js

@@ -21,6 +21,7 @@ function h(tag, props, ...kids) {
 function icon(svg) { const t = document.createElement('template'); t.innerHTML = svg.trim(); return t.content.firstChild; }
 const ICONS = {
   bell: '<svg viewBox="0 0 24 24" width="20" height="20" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M6 8a6 6 0 0 1 12 0c0 7 3 9 3 9H3s3-2 3-9"/><path d="M10.3 21a1.94 1.94 0 0 0 3.4 0"/></svg>',
+  people: '<svg viewBox="0 0 24 24" width="18" height="18" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><circle cx="9" cy="8" r="3.5"/><path d="M2.5 20a6.5 6.5 0 0 1 13 0"/><circle cx="17" cy="9" r="2.5"/><path d="M16 14.2a5 5 0 0 1 6 4.8"/></svg>',
 };
 
 async function api(path, body) {
@@ -50,7 +51,7 @@ function ago(t) {
 }
 
 const qsa = obj => { const p = new URLSearchParams(); for (const k in obj) if (obj[k] != null && obj[k] !== '') p.set(k, obj[k]); return p.toString(); };
-const readUrl = (m, c, v) => 'read.html?' + qsa({ m, c, v });
+const readUrl = (m, c, v, p) => 'read.html?' + qsa({ m, c, v, p });
 const mangaUrl = m => 'manga.html?' + qsa({ m });
 const safeHref = u => (/^https?:\/\//i.test(u || '') ? u : null);