Fix deep link timestamp parsing accepting infinity as seek position (#965)

URLRouter.parseTimestampValue used TimeInterval(_:) for the plain-numeric
branch ("90", "90.5"). That initializer also accepts the special tokens
"inf"/"infinity" and "nan", and the value infinity compares as >= 0, so a
deep link or share URL carrying ?t=inf parsed to Double.infinity and was
forwarded to the player as a seek target. Seeking to infinity is undefined
and breaks playback startup for the affected link.

Reject non-finite values explicitly alongside the existing negative
check, so only finite non-negative seconds are accepted.

- Add isFinite guard to the plain-numeric branch of parseTimestampValue
- Cover inf/infinity/nan/negative rejection in URLRouterTests

Yattee/Services/Navigation/URLRouter.swift

@@ -145,9 +145,11 @@ struct URLRouter: Sendable {
         let trimmed = raw.trimmingCharacters(in: .whitespaces)
         guard !trimmed.isEmpty else { return nil }
 
-        // Plain numeric value (e.g. "90", "90.5")
+        // Plain numeric value (e.g. "90", "90.5"). Reject non-finite values
+        // ("inf"/"infinity"/"nan") that `TimeInterval(_:)` accepts — a `?t=inf`
+        // link would otherwise seek the player to `Double.infinity`.
         if let value = TimeInterval(trimmed) {
-            return value >= 0 ? value : nil
+            return (value.isFinite && value >= 0) ? value : nil
         }
 
         // Compound form like "1h2m3s", "2m30s", "90s"