Add OS keychain for persistent nsec sessions (roadmap #1)

- Rust: store_nsec / load_nsec / delete_nsec Tauri commands via keyring crate
  (macOS Keychain, Windows Credential Manager, Linux Secret Service)
- On nsec login: key is stored in OS keychain keyed by hex pubkey
- On startup: restoreSession() auto-loads nsec from keychain and re-establishes
  the NDK signer — no manual re-login required after restart
- On logout: keychain entry is deleted
- Graceful degradation: if keychain is unavailable (e.g. Linux without a Secret
  Service daemon), the app starts logged-out — same UX as before, no crash

Also updates ROADMAP.md with 4 new items from the Windows playtest (multi-account
switcher, NWC wizard, system tray, zap history view) and reorders the list.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/Cargo.lock

@@ -1807,6 +1807,16 @@ dependencies = [
  "unicode-segmentation",
 ]
 
+[[package]]
+name = "keyring"
+version = "3.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eebcc3aff044e5944a8fbaf69eb277d11986064cba30c468730e8b9909fb551c"
+dependencies = [
+ "log",
+ "zeroize",
+]
+
 [[package]]
 name = "kuchikiki"
 version = "0.8.8-speedreader"
@@ -4983,8 +4993,9 @@ dependencies = [
 
 [[package]]
 name = "wrystr"
-version = "0.1.0"
+version = "0.1.1"
 dependencies = [
+ "keyring",
  "serde",
  "serde_json",
  "tauri",
@@ -5138,6 +5149,12 @@ dependencies = [
  "synstructure",
 ]
 
+[[package]]
+name = "zeroize"
+version = "1.8.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b97154e67e32c85465826e8bcc1c59429aaaf107c1e4a9e53c8d8ccd5eff88d0"
+
 [[package]]
 name = "zerotrie"
 version = "0.2.3"