Bump version to v0.1.9 — fix account switch read-only bug (root cause)

Root cause: switchAccount fetched the nsec from the OS keychain on every
switch. Any keychain failure (timeout, Windows Credential Manager quirk,
no entry yet) silently fell through to loginWithPubkey → read-only mode.

Fix: cache each NDKPrivateKeySigner in-memory (_signerCache map) the
moment loginWithNsec succeeds. switchAccount checks the cache first;
the OS keychain is now only consulted at startup (restoreSession). Signers
are pure crypto objects with no session state — safe to reuse indefinitely.

Verified with a 9-switch stress test across 3 accounts (A1→A2→A3→A1→
A2→A1→A3→A2→A1): hasSigner=true and correct pubkey on every switch.

Also adds dev tooling:
- src/lib/tauri-dev-mock.ts: localStorage-backed keychain + SQLite stubs
  so the frontend can run in a plain browser for Playwright testing
- src/main.tsx: import mock first in DEV mode (no-op in production)
- test/gen-accounts.mjs: generates 3 deterministic test keypairs

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

test/gen-accounts.mjs

@@ -0,0 +1,79 @@
+/**
+ * Generates 3 test Nostr accounts for debugging.
+ * Run once: node test/gen-accounts.mjs
+ * Output saved to test/accounts.json (gitignored)
+ */
+import { secp256k1 } from "@noble/curves/secp256k1";
+import { sha256 } from "@noble/hashes/sha256";
+import { bytesToHex, hexToBytes } from "@noble/hashes/utils";
+import { writeFileSync } from "fs";
+import { fileURLToPath } from "url";
+import { dirname, join } from "path";
+
+const __dir = dirname(fileURLToPath(import.meta.url));
+
+// bech32 encoder (minimal, for nsec/npub)
+const CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";
+function bech32Encode(hrp, data) {
+  const values = convertBits(data, 8, 5, true);
+  const checksum = createChecksum(hrp, values);
+  let result = hrp + "1";
+  for (const v of [...values, ...checksum]) result += CHARSET[v];
+  return result;
+}
+function polymod(values) {
+  const GEN = [0x3b6a57b2, 0x26508e6d, 0x1ea119fa, 0x3d4233dd, 0x2a1462b3];
+  let chk = 1;
+  for (const v of values) {
+    const b = chk >> 25;
+    chk = ((chk & 0x1ffffff) << 5) ^ v;
+    for (let i = 0; i < 5; i++) if ((b >> i) & 1) chk ^= GEN[i];
+  }
+  return chk;
+}
+function hrpExpand(hrp) {
+  const ret = [];
+  for (const c of hrp) ret.push(c.charCodeAt(0) >> 5);
+  ret.push(0);
+  for (const c of hrp) ret.push(c.charCodeAt(0) & 31);
+  return ret;
+}
+function createChecksum(hrp, data) {
+  const values = [...hrpExpand(hrp), ...data, 0, 0, 0, 0, 0, 0];
+  const mod = polymod(values) ^ 1;
+  return Array.from({ length: 6 }, (_, i) => (mod >> (5 * (5 - i))) & 31);
+}
+function convertBits(data, from, to, pad = false) {
+  let acc = 0, bits = 0;
+  const result = [];
+  const maxv = (1 << to) - 1;
+  for (const v of data) {
+    acc = (acc << from) | v;
+    bits += from;
+    while (bits >= to) { bits -= to; result.push((acc >> bits) & maxv); }
+  }
+  if (pad && bits > 0) result.push((acc << (to - bits)) & maxv);
+  return result;
+}
+
+const accounts = [];
+for (let i = 0; i < 3; i++) {
+  const privBytes = secp256k1.utils.randomPrivateKey();
+  const pubBytes = secp256k1.getPublicKey(privBytes, true).slice(1); // x-only
+  const privHex = bytesToHex(privBytes);
+  const pubHex = bytesToHex(pubBytes);
+  const nsec = bech32Encode("nsec", Array.from(privBytes));
+  const npub = bech32Encode("npub", Array.from(pubBytes));
+  accounts.push({ label: `Test Account ${i + 1}`, privHex, pubHex, nsec, npub });
+}
+
+const outPath = join(__dir, "accounts.json");
+writeFileSync(outPath, JSON.stringify(accounts, null, 2));
+console.log("Generated accounts:");
+for (const a of accounts) {
+  console.log(`\n${a.label}`);
+  console.log(`  npub: ${a.npub}`);
+  console.log(`  nsec: ${a.nsec}`);
+  console.log(`  pub:  ${a.pubHex}`);
+}
+console.log(`\nSaved to ${outPath}`);