feat(theming): wire the three navs and add the admin nav builder

Phases 6-8 of docs/website/THEMING_AND_NAV.md. The public header, the admin
sidebar and the player portal now read their override row, and /admin/navigation
writes them: rename, reorder by drag, hide, and — on the admin sidebar — move a
row into another existing section.

The merge always runs BEFORE the role and shard-feature filters in the layouts,
which are unchanged and remain the boundary. An override is presentation: it
cannot introduce a route, cannot touch a `roles` or `feature` gate, and a stored
`hidden: false` on a gated item shows nobody anything.

The design scoped these phases as client work, but the server had no way to
store a nav row: updateSettings validates and stringifies theme_visual and
brand_assets and lets everything else through, so a nav object would have been
written as "[object Object]" and read as absent for ever. utils/navOverrides.js
mirrors utils/brandAssets.js — strict on write with the offending key named,
forgiving on read. It validates shape only; whether a `to` exists is settled
client-side at merge time, because the base NAV arrays are client constants and
a server-side copy would be a second source of truth that drifts.

The nav editor cannot be hidden — its own toggle is disabled, the write path
drops `hidden` on that one `to`, and AdminLayout strips it again before merging,
which also covers a row edited straight in the database.

Orders are written only when the sequence actually differs from the code's, and
the comparison is restricted to the rows the editing admin can see, so renaming
one item does not pin the position of every other one and a role- or
feature-gated item missing from their palette is not mistaken for a reorder.

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

server/src/router/v1/admin/admin.controller.js

@@ -14,6 +14,7 @@ const { cleanBody } = require('../../../utils/sanitizeHtml')
 const { parseJsonSetting } = require('../../../utils/settingsJson')
 const { validateThemeVisual } = require('../../../utils/themeResolve')
 const { validateBrandAssets, resolveBrandAssets } = require('../../../utils/brandAssets')
+const { validateNavOverrides, resolveNavOverrides, NAV_KEYS } = require('../../../utils/navOverrides')
 const htmlShell = require('../../../utils/htmlShell')
 
 const log = require('../../../utils/logger')('admin')
@@ -566,6 +567,23 @@ async function updateSettings(req, res) {
     if (!check.ok) return res.status(400).json({ message: check.message })
     updates.brand_assets = JSON.stringify(resolveBrandAssets(parsed))
   }
+  // The three nav rows are JSON too, and without this they would reach
+  // settingsDb.set as objects and be stored as the string "[object Object]".
+  // Shape only — whether a key names a route the nav actually declares is the
+  // client's question, and utils/navOverrides.js says why. Resolved on the way
+  // in so the stored row carries no dead fields, and so `hidden` can never land
+  // on the nav editor's own row.
+  for (const key of NAV_KEYS) {
+    if (!(key in updates)) continue
+    const raw = updates[key]
+    const parsed = typeof raw === 'string' ? parseJsonSetting(raw) : raw
+    if (typeof raw === 'string' && parsed === null) {
+      return res.status(400).json({ message: `${key} must be a JSON object` })
+    }
+    const check = validateNavOverrides(parsed, key)
+    if (!check.ok) return res.status(400).json({ message: check.message })
+    updates[key] = JSON.stringify(resolveNavOverrides(parsed, key))
+  }
   try {
     await settings.setMany(updates, req.user.id)
     // The HTML shell is templated from brand_assets and theme_visual, and is