feat: I2P seeds for DNS3+Hetzner, Qt I2P panel, snapshot signing

I2P Seed Nodes (#3):
- DNS3: hvvr2yys3nll4l6fdywecvn3baw6h5i7bsa2ldbz2e5xwangnn7q.b32.i2p
- Hetzner: 2hyeunnkax5du4snip4gdsdicxtmlnagtlkatv57rjpx2kvfssma.b32.i2p
- 3 I2P seed nodes now (DNS2 + DNS3 + Hetzner)

Qt Wallet I2P Status (#4):
- Purple [I2P] indicator in status bar (active/building/inactive states)
- .b32.i2p address display alongside .onion address
- Updates every 5s via timer

UTXO Snapshot Signing (#11):
- Ed25519 signature field in SnapshotManifest
- VerifyManifest checks signature when present
- Unsigned manifests get a warning but continue (backward compat)
- Placeholder pubkey — replace when signing key is deployed

src/i2p/i2pseed.h

@@ -15,6 +15,10 @@ static const char *strMainNetI2PSeed[][1] = {
     // DNS2 - primary bootstrap server (194.233.88.206)
     // Generated by embedded i2pd on first run, keys persist in i2p_data/
     {"hnupgkbtcn4hlo6sunhbp6uuz4k6bkgsa5jtcruyyt7y6q7qsoda.b32.i2p"},
+    // DNS3 - canonical chain reference (74.208.167.19)
+    {"hvvr2yys3nll4l6fdywecvn3baw6h5i7bsa2ldbz2e5xwangnn7q.b32.i2p"},
+    // Hetzner Helsinki - ARM64 staking node (46.62.249.20)
+    {"2hyeunnkax5du4snip4gdsdicxtmlnagtlkatv57rjpx2kvfssma.b32.i2p"},
     {nullptr}
 };