Update nostr to v1.0.7 with cross-platform crypto support

- Bump git.mleku.dev/mleku/nostr from v1.0.4 to v1.0.7
- Add p256k1.mleku.dev as indirect dependency for pure Go crypto
- Remove local replace directive for CI compatibility
- Add WASM/Mobile build plan documentation
- Bump version to v0.31.5

nostr v1.0.7 changes:
- Split crypto/p8k into platform-specific files
- Linux uses libsecp256k1 via purego (fast)
- Other platforms (darwin, windows, android) use pure Go p256k1
- Enables cross-compilation without CGO or native libraries

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

go.mod

@@ -3,7 +3,7 @@ module next.orly.dev
 go 1.25.3
 
 require (
-	git.mleku.dev/mleku/nostr v1.0.4
+	git.mleku.dev/mleku/nostr v1.0.7
 	github.com/adrg/xdg v0.5.3
 	github.com/dgraph-io/badger/v4 v4.8.0
 	github.com/dgraph-io/dgo/v230 v230.0.1
@@ -82,6 +82,7 @@ require (
 	google.golang.org/genproto v0.0.0-20200526211855-cb27e3aa2013 // indirect
 	google.golang.org/protobuf v1.36.10 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
+	p256k1.mleku.dev v1.0.3 // indirect
 )
 
 retract v1.0.3