Allow non-null assertions in tests; refactor two production hotspots to drop them

- eslint.config.js: disable no-non-null-assertion for *.test.ts/tsx and
  tests/** (fixture-driven tests routinely use arr[0]! after a length
  check — signal there is low).
- closestFlight.ts: replace flights.legs[0]! / flights[flights.length-1]!
  with explicit null checks.
- FlightDetailsAccordion.tsx: refactor transition + meal/service
  branches to use local consts narrowed by a preceding truthy check,
  dropping the `leg.transition!.registration!` patterns.

Warning count: 190 → 65. Remaining warnings are pre-existing production-code
non-null assertions spread across the codebase.

eslint.config.js

@@ -177,4 +177,16 @@ export default [
       ],
     },
   },
+  // Test files get a looser ruleset: non-null assertions are idiomatic
+  // when fixtures guarantee presence (`arr[0]!` after an explicit length
+  // check). Production code keeps the warning; tests don't.
+  {
+    files: [
+      "src/**/*.test.{ts,tsx}",
+      "tests/**/*.{ts,tsx}",
+    ],
+    rules: {
+      "@typescript-eslint/no-non-null-assertion": "off",
+    },
+  },
 ];