agent term: add a setting to hide debug-level output

hyperhive#4008, mara: 'add a setting to not display verbose output...
like the grey colored debug stuff.'

Same shape as the existing expand-tool-output preference
(ExpandDetailsSetting.tsx): a new shared/src/prefs.ts key pair
(getHideDebugPref/setHideDebugPref), a new settings-menu-row component
owning its own state (HideDebugSetting.tsx, not a prop threaded through
the shared SettingsMenu component — per mara's earlier review on the
first one, more per-page options as props there is how that component
accumulates cruft), mounted next to ExpandDetailsSetting in Root.tsx.

Row.tsx skips (returns null for, not CSS display:none) any TermMsg
whose level is 'debug' when the pref is set — matches the muted 'debug'
row this issue is about (see docs/web-ui/terminal-rendering.md's Levels
table). Read live per-row, same as expand-details, so toggling applies
to newly streamed rows in an already-open tab without a reload; already
-rendered rows are unaffected either way, same non-retroactive
precedent the existing preference already sets.

frontend/packages/agent/src/Root.tsx

@@ -12,6 +12,7 @@ import { useApplyMotionOverride } from "@hive/shared/motion-apply.js";
 import { Header } from "./components/Header.js";
 import { MetaNav } from "./components/MetaNav.js";
 import { ExpandDetailsSetting } from "./components/ExpandDetailsSetting.js";
+import { HideDebugSetting } from "./components/HideDebugSetting.js";
 import { StatusChips } from "./components/StatusChips.js";
 import { LiveStream, type LiveStreamHandle } from "./components/LiveStream.js";
 import { HeaderPill } from "./components/HeaderPill.js";
@@ -151,6 +152,7 @@ export function Root() {
       ) : null}
       <SettingsMenu themeKey={THEME_KEY} motionKey={MOTION_KEY}>
         <ExpandDetailsSetting />
+        <HideDebugSetting />
       </SettingsMenu>
     </>
   );