refactor(settings): extract four panels into settings/ modules

static/js/settings.js was 5,721 lines behind a four-name public surface.
static/js/settings/ already existed with dom, registry, search, sidebar,
navigation and lifecycle, so this continues that package rather than
inventing a layout.

Moved verbatim, 407 lines:

  settings/speech.js        initTtsSettings, initSttSettings
  settings/writingStyle.js  initDocumentWritingStyle
  settings/imageModels.js   initImageSettings
  settings/agent.js         initAgentSettings
  settings/api.js           postSettings, lifted from _postSettings

These five were chosen because their only dependencies outside themselves
were el/byId, _postSettings and sortModelIds. Panels with wider reach stay
put: initEmailAccountsSettings, for instance, pulls a 64-declaration closure
covering most of the file, and splitting that is a design change rather than
a move.

settings.js keeps its public surface exactly: open, close,
refreshAiModelEndpoints and the default export, verified in a browser.

Three test updates the move required:

- tests/helpers/test_settings_shell_coordinator.mjs allowlists the real
  modules the coordinator may import, and rejected the new ones. That guard
  working is the reason to trust the rest of this diff.
- two source-introspection tests read settings.js for behaviour that now
  lives in a panel module. They read the whole settings surface now, so the
  next extraction does not break them again.

static/js/settings/agent.js

@@ -0,0 +1,58 @@
+// Agent settings panel.
+// Extracted verbatim from static/js/settings.js. Behaviour is unchanged: the
+// functions moved, their bodies did not. settings.js imports them and calls
+// them from the same places, so load order and init sequence are untouched.
+
+import { byId as el } from './dom.js';
+import { postSettings as _postSettings } from './api.js';
+
+export async function initAgentSettings() {
+  var toolsInput = el('set-agentMaxTools');
+  var roundsInput = el('set-agentMaxRounds');
+  var supInput = el('set-agentSupervisorLadder');
+  var msg = el('set-agentMsg');
+  if (!toolsInput) return;
+
+  try {
+    var res = await fetch('/api/auth/settings', { credentials: 'same-origin' });
+    var settings = await res.json();
+    if (settings.agent_max_tool_calls) toolsInput.value = settings.agent_max_tool_calls;
+    if (roundsInput && settings.agent_max_rounds) roundsInput.value = settings.agent_max_rounds;
+    if (supInput) supInput.checked = !!settings.agent_supervisor_ladder;
+  } catch (e) {}
+
+  // Clamp + coerce a raw input to an int in [lo, hi]; falls back to `dflt`
+  // when blank/non-numeric. Mirrors the server-side validation.
+  function clampInt(raw, lo, hi, dflt) {
+    var n = parseInt(raw, 10);
+    if (isNaN(n)) return dflt;
+    return Math.max(lo, Math.min(n, hi));
+  }
+
+  async function save() {
+    var tools = clampInt(toolsInput.value, 0, 1000, 0);
+    var rounds = roundsInput ? clampInt(roundsInput.value, 1, 200, 20) : null;
+    toolsInput.value = tools;                       // reflect the clamped value
+    if (roundsInput) roundsInput.value = rounds;
+    var payload = { agent_max_tool_calls: tools };
+    if (rounds != null) payload.agent_max_rounds = rounds;
+    if (supInput) payload.agent_supervisor_ladder = !!supInput.checked;
+    try {
+      await _postSettings(payload);
+      msg.textContent = (tools > 0 ? 'Limit: ' + tools + ' tool calls' : 'Unlimited tool calls') +
+        (rounds != null ? ' · ' + rounds + ' steps/message' : '') +
+        (supInput && supInput.checked ? ' · supervisor on' : '');
+      msg.style.color = 'var(--fg)';
+    } catch (e) { msg.textContent = 'Failed to save'; msg.style.color = 'var(--red)'; }
+  }
+
+  toolsInput.addEventListener('change', save);
+  if (roundsInput) roundsInput.addEventListener('change', save);
+  if (supInput) supInput.addEventListener('change', save);
+  var cur = parseInt(toolsInput.value, 10) || 0;
+  var curR = roundsInput ? (parseInt(roundsInput.value, 10) || 20) : null;
+  msg.textContent = (cur > 0 ? 'Limit: ' + cur + ' tool calls' : 'Unlimited tool calls') +
+    (curR != null ? ' · ' + curR + ' steps/message' : '') +
+    (supInput && supInput.checked ? ' · supervisor on' : '');
+
+}