`build_flight_booking_url` hardcodes economy — booking links open in the wrong cabin

[drphilth]

Every deep-link from build_flight_booking_url opens in economy for 1 passenger, regardless of the cabin searched, because the tfs seat-class field is hardcoded to 1 in build_tfs_token with no parameter. Affects the flights command's links too, not just multi. Verified by hand: flipping the field to 3 yields a business-class booking page. (Passenger count has the same issue but its encoding is a nested message — scoping this to cabin and noting pax as a follow-up.)

--- a/src/fli/search/_proto.py
+++ b/src/fli/search/_proto.py
@@ def build_tfs_token(
-def build_tfs_token(segments, *, is_one_way: bool = True) -> str:
+def build_tfs_token(segments, *, is_one_way: bool = True, seat: int = 1) -> str:
@@ payload = (
-        + _varint_field(9, 1)
+        + _varint_field(9, seat)   # 1=economy 2=premium 3=business 4=first
--- a/src/fli/search/flights.py
+++ b/src/fli/search/flights.py
@@ def build_flight_booking_url(self, flight, *, currency=None, language=None, country=None):
+        # seat_type: SeatType (ECONOMY=1 … FIRST=4); default economy for back-compat
-            tfs = build_tfs_token(segments, is_one_way=is_one_way)
+            tfs = build_tfs_token(segments, is_one_way=is_one_way, seat=seat_type.value)

Plus: add seat_type: SeatType = SeatType.ECONOMY to build_flight_booking_url's signature, and have the callers (cli/commands/flights.py, mcp/server.py::_execute_search) pass the search's seat_type through.