Arm the watchdog for every mode, not just snapshot

The watchdog was spawned inside snapshot.run, so legacy highlights mode ran
without it. That is the mode NickelMenu invokes, against the endpoint that
stopped responding and hung a run for 16 days — so the exact failure being
fixed was still reachable from the device's menu.

Moves the spawn into main, ahead of mode dispatch, so both paths are covered.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/snapshot.zig

@@ -35,6 +35,13 @@ fn watchdog(secs: u64) void {
     std.process.exit(2);
 }
 
+/// Arm the watchdog for the current process. Every mode needs this, not just snapshot:
+/// legacy highlights mode POSTs to the same kind of endpoint and hangs the same way.
+pub fn startWatchdog(secs: u64) !void {
+    const guard = try std.Thread.spawn(.{}, watchdog, .{secs});
+    guard.detach();
+}
+
 fn sha256Hex(data: []const u8, out: *[64]u8) void {
     var digest: [32]u8 = undefined;
     std.crypto.hash.sha2.Sha256.hash(data, &digest, .{});
@@ -97,16 +104,13 @@ fn writeSnapshot(allocator: std.mem.Allocator, db_path: []const u8, snap_path: [
     if (c.sqlite3_close(db) != c.SQLITE_OK) return error.DatabaseCloseFailed;
 }
 
+/// Caller is expected to have armed `startWatchdog` already.
 pub fn run(
     allocator: std.mem.Allocator,
     db_path: []const u8,
     endpoint: ?[]const u8,
     snap_path: []const u8,
-    timeout_secs: u64,
 ) !void {
-    const guard = try std.Thread.spawn(.{}, watchdog, .{timeout_secs});
-    guard.detach();
-
     try writeSnapshot(allocator, db_path, snap_path);
     // Database handle is released from here on.