feat(webui): internet sharing UI — exit-offer toggle + peer route-via selector

CellNetwork page (CellPanel):
- Internet Sharing section below service toggles
- Toggle: 'Offer my internet to <cell>' (calls PUT /api/cells/<n>/exit-offer)
- Read-only indicator: whether remote cell offers internet back
- Contextual hints explaining what each party needs to do next

Peers page:
- Fetches connected cells on mount
- Edit modal: Internet Exit dropdown (route-via) showing all connected cells
  with ✓ marker for cells that have offered internet
- Warning if selected cell hasn't offered internet yet
- On save, calls PUT /api/peers/<n>/route-via only when value changed
- Table badge shows 'via <cell>' for peers with active routing

api.js:
- cellLinkAPI.setExitOffer(cellName, offered)
- peerRegistryAPI.setRouteVia(peerName, viaCell)

Tests (vitest + @testing-library/react):
- 19 new frontend tests in src/__tests__/
  - CellNetworkInternetSharing.test.jsx (10 tests)
  - PeersRouteVia.test.jsx (9 tests)
- make test-webui target runs them via docker node:18-alpine

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

webui/src/pages/CellNetwork.jsx

@@ -1,5 +1,5 @@
 import { useState, useEffect } from 'react';
-import { Link2, Link2Off, Copy, CheckCheck, RefreshCw, Plug, Unplug, Globe, Wifi, Calendar, FolderOpen, Mail, HardDrive, ChevronDown, ChevronRight } from 'lucide-react';
+import { Link2, Link2Off, Copy, CheckCheck, RefreshCw, Plug, Unplug, Globe, Wifi, Calendar, FolderOpen, Mail, HardDrive, ChevronDown, ChevronRight, ArrowUpFromLine } from 'lucide-react';
 import { cellLinkAPI } from '../services/api';
 import { useConfig } from '../contexts/ConfigContext';
 import QRCode from 'qrcode';
@@ -143,6 +143,8 @@ function CellPanel({ conn, onDisconnect, addToast }) {
   const [inboundPerms, setInboundPerms] = useState(conn.permissions?.inbound || {});
   const [saving, setSaving] = useState({});
   const [confirmDisconnect, setConfirmDisconnect] = useState(false);
+  const [exitOffered, setExitOffered] = useState(!!conn.exit_offered);
+  const [savingExit, setSavingExit] = useState(false);
 
   const handleToggle = async (serviceKey, newValue) => {
     setSaving(s => ({ ...s, [serviceKey]: true }));
@@ -158,6 +160,19 @@ function CellPanel({ conn, onDisconnect, addToast }) {
     }
   };
 
+  const handleExitToggle = async (newValue) => {
+    setSavingExit(true);
+    try {
+      await cellLinkAPI.setExitOffer(conn.cell_name, newValue);
+      setExitOffered(newValue);
+      addToast(`Internet sharing ${newValue ? 'offered' : 'withdrawn'}`);
+    } catch {
+      addToast('Failed to update internet sharing', 'error');
+    } finally {
+      setSavingExit(false);
+    }
+  };
+
   const hasRevokedService = Object.values(inboundPerms).some(v => !v);
 
   return (
@@ -246,6 +261,39 @@ function CellPanel({ conn, onDisconnect, addToast }) {
                 </p>
               </div>
             </div>
+            {/* ── Internet sharing ───────────────────────────────────── */}
+            <div className="mt-4 pt-4 border-t border-gray-100">
+              <p className="text-xs font-semibold text-gray-500 uppercase tracking-wide mb-3 flex items-center gap-1.5">
+                <ArrowUpFromLine className="h-3.5 w-3.5" /> Internet Sharing
+              </p>
+              <div className="flex flex-col sm:flex-row sm:items-center gap-4">
+                <ServiceShareToggle
+                  serviceKey="internet"
+                  label={`Offer my internet to ${conn.cell_name}`}
+                  Icon={Globe}
+                  enabled={exitOffered}
+                  saving={savingExit}
+                  onChange={handleExitToggle}
+                />
+                <div className={`flex items-center gap-1.5 text-sm ${conn.remote_exit_offered ? 'text-green-700' : 'text-gray-400'}`}>
+                  <span className={`inline-block h-2 w-2 rounded-full flex-shrink-0 ${conn.remote_exit_offered ? 'bg-green-500' : 'bg-gray-300'}`} />
+                  {conn.remote_exit_offered
+                    ? <>{conn.cell_name} offers internet</>
+                    : <>{conn.cell_name} doesn't offer internet</>}
+                </div>
+              </div>
+              {exitOffered && (
+                <p className="text-xs text-gray-400 mt-2">
+                  The {conn.cell_name} admin can route their peers' internet through your connection via Peers → Edit peer → Internet Exit.
+                </p>
+              )}
+              {conn.remote_exit_offered && (
+                <p className="text-xs text-gray-400 mt-2">
+                  You can route any local peer's internet through {conn.cell_name} via Peers → Edit peer → Internet Exit.
+                </p>
+              )}
+            </div>
+
             <div className="mt-4 pt-3 border-t border-gray-100 flex flex-wrap items-center justify-between gap-3">
               <dl className="flex flex-wrap gap-x-4 gap-y-1 text-xs text-gray-500">
                 {conn.vpn_subnet && <div><dt className="inline text-gray-400">Subnet </dt><dd className="inline font-mono">{conn.vpn_subnet}</dd></div>}