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.sum

@@ -1,6 +1,6 @@
 cloud.google.com/go v0.26.0/go.mod h1:aQUYkXzVsufM+DwF1aE+0xfcU+56JwCaLick0ClmMTw=
-git.mleku.dev/mleku/nostr v1.0.4 h1:QKJlqUubLPeMpYpxHODSvfSlL+F6UhjBiBuze9FGRKo=
-git.mleku.dev/mleku/nostr v1.0.4/go.mod h1:swI7bWLc7yU1jd7PLCCIrIcUR3Ug5O+GPvpub/w6eTY=
+git.mleku.dev/mleku/nostr v1.0.7 h1:BXWsAAiGu56JXR4rIn0kaVOE+RtMmA9MPvAs8y/BjnI=
+git.mleku.dev/mleku/nostr v1.0.7/go.mod h1:iYTlg2WKJXJ0kcsM6QBGOJ0UDiJidMgL/i64cHyPjZc=
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
 github.com/BurntSushi/toml v1.5.0 h1:W5quZX/G/csjUnuI8SUYlsHs9M38FC7znL0lIO+DvMg=
 github.com/BurntSushi/toml v1.5.0/go.mod h1:ukJfTF/6rtPPRCnwkur4qwRxa8vTRFBF0uk2lLoLwho=
@@ -302,3 +302,5 @@ lol.mleku.dev v1.0.5/go.mod h1:JlsqP0CZDLKRyd85XGcy79+ydSRqmFkrPzYFMYxQ+zs=
 lukechampine.com/frand v1.5.1 h1:fg0eRtdmGFIxhP5zQJzM1lFDbD6CUfu/f+7WgAZd5/w=
 lukechampine.com/frand v1.5.1/go.mod h1:4VstaWc2plN4Mjr10chUD46RAVGWhpkZ5Nja8+Azp0Q=
 nullprogram.com/x/optparse v1.0.0/go.mod h1:KdyPE+Igbe0jQUrVfMqDMeJQIJZEuyV7pjYmp6pbG50=
+p256k1.mleku.dev v1.0.3 h1:2SBEH9XhNAotO1Ik8ejODjChTqc06Z/6ncQhrYkAdRA=
+p256k1.mleku.dev v1.0.3/go.mod h1:cWkZlx6Tu7CTmIxonFbdjhdNfkY3VbjjY5TFEILiTnY=