Fix description timestamp links seeking to wrong position for out-of-range values (#966)

DescriptionText.parseTimestamp turned a matched timestamp string into a
clickable seek link by computing seconds from its colon-separated parts
with no range validation. The link regex (\d{1,2}:\d{2}(?::\d{2})?)
matches strings that are not valid clock positions, such as 1:99 or
0:60, and parseTimestamp happily computed 1*60+99 = 159 for 1:99 — so
tapping such a link seeked the player to 2:39 instead of being ignored.

This also diverged from ChapterParser, which rejects seconds/minutes
>= 60; the same timestamp string could be dropped as a chapter but
still seek as a description link.

- Validate seconds < 60 in the MM:SS branch and minutes < 60, seconds
  < 60 in the H:MM:SS branch; return nil otherwise
- Change parseTimestamp to return Int? and skip building the seek link
  for nil, so out-of-range matches are left as plain text
- Add DescriptionTextTests with valid, boundary, and out-of-range cases

YatteeTests/DescriptionTextTests.swift

@@ -0,0 +1,67 @@
+//
+//  DescriptionTextTests.swift
+//  YatteeTests
+//
+//  Unit tests for description-text timestamp parsing.
+//
+
+import Testing
+import Foundation
+@testable import Yattee
+
+@Suite("DescriptionText Timestamp Parsing")
+struct DescriptionTextTimestampTests {
+
+    // MARK: - Valid Timestamps
+
+    @Test("Parses M:SS format")
+    func parseMSS() {
+        #expect(DescriptionText.parseTimestamp("0:00") == 0)
+        #expect(DescriptionText.parseTimestamp("5:30") == 330)
+    }
+
+    @Test("Parses MM:SS format")
+    func parseMMSS() {
+        #expect(DescriptionText.parseTimestamp("00:00") == 0)
+        #expect(DescriptionText.parseTimestamp("12:45") == 765)
+        #expect(DescriptionText.parseTimestamp("59:59") == 3599)
+    }
+
+    @Test("Parses H:MM:SS format")
+    func parseHMMSS() {
+        #expect(DescriptionText.parseTimestamp("1:23:45") == 5025)
+        #expect(DescriptionText.parseTimestamp("01:23:45") == 5025)
+    }
+
+    @Test("Parses long minutes in MM:SS form")
+    func parseLongMinutes() {
+        // A 99-minute position is valid for a long video (2-digit minutes).
+        #expect(DescriptionText.parseTimestamp("99:59") == 5999)
+    }
+
+    // MARK: - Out-of-Range Rejection (regression)
+
+    @Test("Rejects seconds >= 60 in MM:SS form")
+    func rejectsInvalidSeconds() {
+        // Previously returned components[0]*60 + components[1] = 60 and 159,
+        // seeking the player to the wrong position.
+        #expect(DescriptionText.parseTimestamp("0:60") == nil)
+        #expect(DescriptionText.parseTimestamp("1:99") == nil)
+        #expect(DescriptionText.parseTimestamp("5:75") == nil)
+    }
+
+    @Test("Rejects minutes or seconds >= 60 in H:MM:SS form")
+    func rejectsInvalidHoursMinutesSeconds() {
+        #expect(DescriptionText.parseTimestamp("1:99:30") == nil)
+        #expect(DescriptionText.parseTimestamp("1:30:60") == nil)
+        #expect(DescriptionText.parseTimestamp("2:60:00") == nil)
+    }
+
+    // MARK: - Boundary
+
+    @Test("Accepts the 59 boundary in every field")
+    func acceptsBoundary() {
+        #expect(DescriptionText.parseTimestamp("59:59") == 3599)
+        #expect(DescriptionText.parseTimestamp("1:59:59") == 7199)
+    }
+}