fix(api): send YouTube's utcOffsetMinutes sign, fixing YTMusicClient

Sources/APIExplorer/AskVideoAudit.swift

@@ -399,6 +399,10 @@ private func askParityContext(
         "osVersion": "10_15_7",
         "platform": "DESKTOP",
         "userAgent": askParityUserAgent,
+        // Minutes from UTC, UTC-behind negative — the same value YouTube's web client
+        // sends (its JS `-Date.getTimezoneOffset()`). Mirrors
+        // `InnerTubeSupport.utcOffsetMinutes(for:)`, inlined because APIExplorer cannot
+        // import the Kaset executable target.
         "utcOffsetMinutes": TimeZone.current.secondsFromGMT() / 60,
     ]
     if profile.usesVisitorData {

Sources/Kaset/Services/API/InnerTubeSupport.swift

@@ -19,4 +19,13 @@ enum InnerTubeSupport {
             .joined()
         return "\(timestamp)_\(hash)"
     }
+
+    /// The `client.utcOffsetMinutes` value for an InnerTube request: minutes offset
+    /// from UTC, UTC-behind zones NEGATIVE (Los Angeles is `-420`). This matches what
+    /// YouTube's own web client sends — its JS computes `-Date.getTimezoneOffset()`,
+    /// and `TimeZone.secondsFromGMT()` already carries that opposite sign, so no
+    /// negation is needed here.
+    static func utcOffsetMinutes(for timeZone: TimeZone) -> Int {
+        timeZone.secondsFromGMT() / 60
+    }
 }

Sources/Kaset/Services/API/YTMusicClient.swift

@@ -1959,7 +1959,7 @@ final class YTMusicClient: YTMusicClientProtocol {
                 "osVersion": "10_15_7",
                 "platform": "DESKTOP",
                 "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Safari/605.1.15",
-                "utcOffsetMinutes": -TimeZone.current.secondsFromGMT() / 60,
+                "utcOffsetMinutes": InnerTubeSupport.utcOffsetMinutes(for: .current),
             ],
             "user": userDict,
         ]

Sources/Kaset/Services/API/YouTubeClient.swift

@@ -689,7 +689,7 @@ final class YouTubeClient: YouTubeClientProtocol { // swiftlint:disable:this typ
                 "osVersion": "10_15_7",
                 "platform": "DESKTOP",
                 "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Safari/605.1.15",
-                "utcOffsetMinutes": TimeZone.current.secondsFromGMT() / 60,
+                "utcOffsetMinutes": InnerTubeSupport.utcOffsetMinutes(for: .current),
             ],
             "user": userDict,
         ]

Tests/KasetTests/InnerTubeSupportTests.swift

@@ -50,4 +50,42 @@ struct InnerTubeSupportTests {
         )
         #expect(hash.hasPrefix("42_"))
     }
+
+    @Test("utcOffsetMinutes is negative for a UTC-behind zone (matches YouTube web)")
+    func utcOffsetMinutesBehindUTC() throws {
+        // Fixed UTC-5 offset (DST-independent) so the assertion is stable on any
+        // run date; secondsFromGMT() == -18_000 → -18_000 / 60 == -300.
+        let tz = try #require(TimeZone(secondsFromGMT: -18000))
+        #expect(InnerTubeSupport.utcOffsetMinutes(for: tz) == -300)
+    }
+
+    @Test("utcOffsetMinutes is positive for a UTC-ahead zone (matches YouTube web)")
+    func utcOffsetMinutesAheadOfUTC() throws {
+        // Fixed UTC+1 offset (DST-independent); secondsFromGMT() == 3600 → 3600 / 60 == 60.
+        let tz = try #require(TimeZone(secondsFromGMT: 3600))
+        #expect(InnerTubeSupport.utcOffsetMinutes(for: tz) == 60)
+    }
+
+    @Test("utcOffsetMinutes is zero for UTC")
+    func utcOffsetMinutesUTC() throws {
+        let tz = try #require(TimeZone(identifier: "UTC"))
+        #expect(InnerTubeSupport.utcOffsetMinutes(for: tz) == 0)
+    }
+
+    @Test("utcOffsetMinutes carries the fractional half-hour sign for India")
+    func utcOffsetMinutesHalfHour() throws {
+        let tz = try #require(TimeZone(identifier: "Asia/Kolkata")) // IST UTC+5:30 (no DST) → +19_800s
+        #expect(InnerTubeSupport.utcOffsetMinutes(for: tz) == 330)
+    }
+
+    @Test("utcOffsetMinutes pins the Los Angeles datum YouTube's web client sends")
+    func utcOffsetMinutesLosAngelesReference() throws {
+        // YouTube's web client sends `-Date.getTimezoneOffset()` (YouTube.js,
+        // Session.ts), which equals `secondsFromGMT() / 60` in Foundation's opposite
+        // sign convention. Los Angeles on daylight time is UTC-7: -25_200s → -420,
+        // the datum in the review discussion. A fixed offset is used rather than the
+        // named zone so DST cannot move the assertion.
+        let tz = try #require(TimeZone(secondsFromGMT: -25200))
+        #expect(InnerTubeSupport.utcOffsetMinutes(for: tz) == -420)
+    }
 }