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.

docs/web-ui/terminal-rendering.md

@@ -35,6 +35,11 @@ leading the summary text rather than the icon.
 | `warn` | amber, left rule | stderr, an unrecognised event shape, API retries |
 | `error` | red, left rule | turn failed, a tool result with `is_error: true` |
 
+The settings menu's "hide debug output" toggle (`getHideDebugPref()`,
+`@hive/shared/prefs.js`) skips `debug`-level rows entirely rather than
+muting them further — client-side only, same live-read-no-reload shape
+as the expand-tool-output preference above.
+
 Per-tool icon + summary formatting (what a `Read`/`Edit`/`send` call's row
 actually says) lives in `stream_enrich.rs`'s `fmt_tool_use()` family —
 read that when you need the specifics, this doc doesn't duplicate it.