feat: Lightning channel backup, Web5 mobile tab active, file path fix

Task 14: Lightning Channel Backup
- New lnd.export-channel-backup RPC — exports SCB (Static Channel Backup)
- Settings UI: "Lightning Channel Backup" section with export + copy
- Returns base64 backup data, channel count, timestamp

Web5 mobile tab active state
- Fixed combined tab matching for Web5: includes /web5, /federation, /mesh routes
- Previously only matched /cloud and /server (wrong branch)

Content file path fix
- Allow forward slashes in filenames for subdirectories (Music/song.mp3)
- Still block .., \, null bytes, hidden files, absolute paths

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

neode-ui/src/views/Dashboard.vue

@@ -304,7 +304,9 @@
             'nav-tab-active': item.isCombined
               ? (item.path === '/dashboard/apps'
                   ? (route.path.includes('/apps') || route.path.includes('/marketplace') || route.path.includes('/discover') || route.path.includes('/app-session'))
-                  : (route.path.includes('/cloud') || route.path.includes('/server')))
+                  : item.path === '/dashboard/web5'
+                    ? (route.path.includes('/web5') || route.path.includes('/federation') || route.path.includes('/mesh'))
+                    : (route.path.includes('/cloud') || route.path.includes('/server')))
               : undefined
           }"
           :exact-active-class="item.isCombined ? undefined : 'nav-tab-active'"