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

YatteeTests/NavigationTests.swift

@@ -123,6 +123,20 @@ struct URLRouterTests {
         #expect(URLRouter.parseTimestampValue("90.5") == 90.5)
     }
 
+    @Test("Reject non-finite and negative plain timestamp values")
+    func rejectInvalidPlainTimestampValues() {
+        // Plain numeric values that are accepted by TimeInterval(_:) but are not
+        // valid seek targets must return nil rather than poisoning the player.
+        #expect(URLRouter.parseTimestampValue("inf") == nil)
+        #expect(URLRouter.parseTimestampValue("infinity") == nil)
+        #expect(URLRouter.parseTimestampValue("nan") == nil)
+        #expect(URLRouter.parseTimestampValue("-5") == nil)
+        // Valid plain seconds still parse.
+        #expect(URLRouter.parseTimestampValue("0") == 0)
+        #expect(URLRouter.parseTimestampValue("90") == 90)
+        #expect(URLRouter.parseTimestampValue("90.5") == 90.5)
+    }
+
     // MARK: - Share Extension Wrapper Tests
 
     @Test("Unwrap yattee://open wrapper to inner URL with timestamp")