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>

core/archipelago/src/api/rpc/mod.rs

@@ -514,6 +514,7 @@ impl RpcHandler {
             "lnd.create-raw-tx" => self.handle_lnd_create_raw_tx(params).await,
             "lnd.gettransactions" => self.handle_lnd_gettransactions().await,
             "lnd.connect-info" => self.handle_lnd_connect_info().await,
+            "lnd.export-channel-backup" => self.handle_lnd_export_channel_backup().await,
 
             // Multi-identity management
             "identity.list" => self.handle_identity_list(params).await,