feat(nav): N1 — collapsible left nav sidebar (Design C)

Move the modules (Feed/Channels/Playlists/Stats) out of the avatar dropdown into a
persistent left sidebar with icon+label entries; collapses to a thin icon rail
(persisted in localStorage). Account actions (About, Sign out, admin badge) move to a
popover at the sidebar bottom; Settings is a rail entry. Header is now a contextual top
bar (sync status, feed scope + search or page title, language, notifications) — logo and
account menu removed from it. New nav.json strings (HU/EN/DE). First step of Epic N.

frontend/src/i18n/locales/de/nav.json

@@ -0,0 +1,5 @@
+{
+  "primary": "Hauptnavigation",
+  "collapse": "Seitenleiste einklappen",
+  "expand": "Seitenleiste ausklappen"
+}

frontend/src/i18n/locales/en/nav.json

@@ -0,0 +1,5 @@
+{
+  "primary": "Primary navigation",
+  "collapse": "Collapse sidebar",
+  "expand": "Expand sidebar"
+}

frontend/src/i18n/locales/hu/nav.json

@@ -0,0 +1,5 @@
+{
+  "primary": "Fő navigáció",
+  "collapse": "Oldalsáv összecsukása",
+  "expand": "Oldalsáv kinyitása"
+}