feat(launcher): add sync command to merge stats and history between machines over SSH

subminer sync <host> exchanges VACUUM INTO snapshots over ssh/scp and each
side merges the other's data as an insert-only union keyed on session UUIDs,
video keys, series title keys, and word/kanji identity. Lifetime totals and
daily/monthly rollups are applied incrementally so pre-retention history
survives, remote-only historical rollups are copied, and re-syncing is
idempotent. sync --snapshot/--merge expose the underlying steps for manual
transfers; a pid-file/mpv-socket guard refuses to run while SubMiner may be
writing the database and schema-version mismatches abort the merge.

launcher/sync/ssh.ts

@@ -0,0 +1,58 @@
+import { spawnSync } from 'node:child_process';
+
+export interface RemoteRunResult {
+  status: number;
+  stdout: string;
+}
+
+/**
+ * Run a command on the SSH host. stdin/stderr stay attached to the terminal
+ * so key passphrase / password prompts and remote progress output work;
+ * stdout is captured for the caller.
+ */
+export function runSsh(host: string, remoteCommand: string): RemoteRunResult {
+  const result = spawnSync('ssh', [host, remoteCommand], {
+    encoding: 'utf8',
+    stdio: ['inherit', 'pipe', 'inherit'],
+  });
+  if (result.error) {
+    throw new Error(`Failed to run ssh: ${(result.error as Error).message}`);
+  }
+  return { status: result.status ?? 1, stdout: result.stdout ?? '' };
+}
+
+export function runScp(from: string, to: string): void {
+  const result = spawnSync('scp', ['-q', from, to], {
+    encoding: 'utf8',
+    stdio: ['inherit', 'inherit', 'inherit'],
+  });
+  if (result.error) {
+    throw new Error(`Failed to run scp: ${(result.error as Error).message}`);
+  }
+  if ((result.status ?? 1) !== 0) {
+    throw new Error(`scp failed copying ${from} -> ${to}`);
+  }
+}
+
+export function shellQuote(value: string): string {
+  return `'${value.replaceAll("'", `'\\''`)}'`;
+}
+
+/**
+ * Non-interactive SSH shells often miss ~/.local/bin in PATH, so probe the
+ * configured command first and fall back to the default install location.
+ */
+export function resolveRemoteSubminerCommand(host: string, preferred: string | null): string {
+  const candidates = preferred ? [preferred] : ['subminer', '~/.local/bin/subminer'];
+  for (const candidate of candidates) {
+    const probe = runSsh(host, `command -v ${candidate} >/dev/null 2>&1`);
+    if (probe.status === 0) {
+      return candidate;
+    }
+  }
+  throw new Error(
+    preferred
+      ? `Remote command not found on ${host}: ${preferred}`
+      : `subminer not found on ${host} (tried PATH and ~/.local/bin/subminer). Pass --remote-cmd <path>.`,
+  );
+}