refactor(i18n): split locale files into per-namespace modules (Phase K) (#688)

Each src/locales/<lang>.ts (~1800 LOC) becomes a folder src/locales/<lang>/
with one module per i18n namespace (44 each) plus an index.ts barrel that
reassembles <lang>Translation in the original key order.

Mechanical, script-driven split with a JSON round-trip check: every
locale object is byte-identical to its pre-split form. i18n.ts is
unchanged — './locales/<lang>' now resolves to the folder index.

The per-namespace settings.ts files land ~440-460 LOC; a single i18n
namespace is the natural, non-arbitrary split unit for a flat string
table, so they are intentionally left whole.

src/locales/en/connection.ts

@@ -0,0 +1,28 @@
+export const connection = {
+  connected: 'Connected',
+  connectedTo: 'Connected to {{server}}',
+  disconnected: 'Disconnected',
+  disconnectedFrom: 'Cannot reach {{server}} — click to check settings',
+  checking: 'Connecting…',
+  extern: 'Extern',
+  offlineTitle: 'No server connection',
+  offlineSubtitle: 'Cannot reach {{server}}. Check your network or server.',
+  offlineModeBanner: 'Offline Mode — playing from local cache',
+  offlineNoCacheBanner: 'No server connection — cannot reach {{server}}',
+  offlineLibraryTitle: 'Offline Library',
+  offlineLibraryEmpty: 'No albums cached yet. Go online, open an album and click "Make available offline".',
+  offlineAlbumCount: '{{n}} album',
+  offlineAlbumCount_plural: '{{n}} albums',
+  offlineFilterAll: 'All',
+  offlineFilterAlbums: 'Albums',
+  offlineFilterPlaylists: 'Playlists',
+  offlineFilterArtists: 'Discographies',
+  retry: 'Retry',
+  serverSettings: 'Server Settings',
+  switchServerTitle: 'Switch server',
+  switchServerHint: 'Click to choose another saved server.',
+  manageServers: 'Manage servers…',
+  switchFailed: 'Could not switch — server unreachable.',
+  lastfmConnected: 'Last.fm connected as @{{user}}',
+  lastfmSessionInvalid: 'Session invalid — click to re-connect',
+};