feat(app): the module registry drives the render (S6b DoD)

Replace App's page-render ternary and the three side-rail page conditionals with
a data-driven lookup: components/moduleRegistry maps each page to a prop-free
content component (+ optional side rail + shell fadeTop), and App renders
PAGE_CONTENT[page]/PAGE_RAIL[page] gated by moduleDef(page).gate (an unreachable
gated page falls back to the feed, as the old ternary did). Content modules and
the two rails are now prop-free: they read `me` via the new useMe() hook and
their module-local state from the scoped providers; FeedPage/MessagesPage are the
only wrappers (they wire the global feed scope / meId). App.tsx 796->431 lines,
zero `page===X` chrome conditionals — closes the Platform epic DoD. (Platform S6b.3)

frontend/src/components/PlaylistsRail.tsx

@@ -2,7 +2,7 @@ import { useEffect, useMemo, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { ArrowDown, ArrowUp, ListVideo, Pin, Plus, RefreshCw, Youtube } from "lucide-react";
-import { api, type Me, type Playlist } from "../lib/api";
+import { api, type Playlist } from "../lib/api";
 import { formatDuration } from "../lib/format";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
@@ -11,6 +11,7 @@ import { defaultLayout, type PanelLayout } from "../lib/panelLayout";
 import SidePanel from "./SidePanel";
 import PanelGroups, { type PanelItem } from "./PanelGroups";
 import { usePlaylists, usePlaylistsActions } from "./PlaylistsProvider";
+import { useMe } from "../lib/useMe";
 
 type PlSort = { key: "custom" | "name" | "count" | "duration"; dir: "asc" | "desc"; dirtyFirst: boolean };
 const PL_SORT_DEFAULT: PlSort = { key: "custom", dir: "asc", dirtyFirst: false };
@@ -20,19 +21,17 @@ const PL_SORT_DEFAULT: PlSort = { key: "custom", dir: "asc", dirtyFirst: false }
 // controls (sort / new / sync); the selected playlist is App state so the module's detail pane
 // (Playlists.tsx) reacts to it. Grouped into two islands: "manage" and the playlist list.
 export default function PlaylistsRail({
-  me,
   layout,
   setLayout,
   collapsed,
   onToggleCollapse,
 }: {
-  me: Me;
   layout: PanelLayout;
   setLayout: (l: PanelLayout) => void;
   collapsed: boolean;
   onToggleCollapse: () => void;
 }) {
-  const canWrite = me.can_write;
+  const canWrite = useMe().can_write;
   // The name filter + the selected playlist live in PlaylistsProvider (shared with the header search
   // box and the Playlists detail pane); this rail owns the list query + its own controls.
   const { search, selectedId } = usePlaylists();