Expand gtest coverage across recently changed modules

Adds ~60 new unit tests covering the modules touched in recent bug-fix
passes and UI refactors:

- RenderTicket: result/finish-count semantics, empty watcher defaults,
  ticket-reuse rejection
- Preferences tabs: behavior options migrated into General/Audio tabs,
  translation context helper, audio-tab initialization
- NodeView: context management, 'Show in Parameter Editor' action wiring
- PreviewAutoCacher: construction, pause toggles, single-frame render
  cancellation, force-cache range
- SeekableWidget/TimeBasedWidget: scroll, marker editing, context reset
- OCIOLutNode: empty/missing/unsupported LUT paths, string direction
  values, file-switching regression coverage
- ProxyManager: working filename, disabled state, empty proxy fields
- Frame/VideoParams/AudioParams/Timecode/ExportFormat/ExportCodec:
  additional edge-case and enumeration coverage

Also fixes the PreferencesAudioTab test crash by initializing
AudioManager in the offscreen test environment.

All tests pass: ctest --output-on-failure

tests/gtest/timecode_metadata_test.cpp

@@ -66,6 +66,23 @@ TEST(TimecodeMetadata, RejectsInvalidMetadata)
 	EXPECT_FALSE(olive::TimecodeMetadata::FromBwfTimeReference(
 					 QStringLiteral("123"), 0)
 					 .valid);
+	EXPECT_FALSE(olive::TimecodeMetadata::FromTimecodeString(
+					 QStringLiteral("not-a-timecode"),
+					 olive::core::rational(1, 24))
+					 .valid);
+}
+
+TEST(TimecodeMetadata, FromBwfTimeReferenceZeroSampleRateIsInvalid)
+{
+	EXPECT_FALSE(olive::TimecodeMetadata::FromBwfTimeReference(
+					 QStringLiteral("0"), 0)
+					 .valid);
+}
+
+TEST(TimecodeMetadata, FootageDescriptionWithoutSourceStartTime)
+{
+	olive::FootageDescription desc(QStringLiteral("ffmpeg"));
+	EXPECT_FALSE(desc.HasSourceStartTime());
 }
 
 TEST(TimecodeMetadata, FootageDescriptionCachesSourceStartTime)