Render the container row for a video nothing could name (#199)

ConverterScreen.kt:668's null arm -- DetailRow("Container", probe.container?.label ?:
"Unknown") in the VIDEO branch -- had never rendered. Every video case in FileCardTest uses
VIDEO_PROBE, which carries container = MP4.

The argument for adding it is the asymmetry, not the coverage. FileCard renders that exact
expression twice, once in AUDIO_ONLY (:660) and once in VIDEO (:668), and "an audio-only
file nothing else could describe degrades one row at a time" drives only the first. Same
expression, same fallback, one kind covered and one not -- which is the same argument
CLAUDE.md records for including ContainerCapabilities:94.

Nor is null an edge case here. InputProbe.container's own KDoc says MediaExtractor cannot
report a container at all, so it comes from FFprobe alone: any run where FFprobe did not
answer produces exactly this shape -- real codec, real dimensions, real duration, no
container. An empty value in its place would read as a rendering bug rather than as a
probe that got half its sources.

The other three rows are asserted alongside, which is what keeps this from being a copy of
the audio-only case. There, everything is unknown at once; here one field is missing from
a probe that is otherwise complete, and the rest have to be unaffected by it.

Mutation: `?: "Unknown"` -> `?: ""` at :668 only, run and restored. The AUDIO_ONLY twin at
:660 is a separate expression, and mutating that one would redden the existing test instead
-- which would prove nothing about this one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/test/java/org/libremediaconverter/convert/FileCardTest.kt

@@ -205,6 +205,34 @@ class FileCardTest {
         assertNoRow("Length")
     }
 
+    /**
+     * A video the app knows a great deal about and cannot name the container of.
+     *
+     * Not an edge case. `InputProbe.container`'s own KDoc says `MediaExtractor` cannot report a
+     * container at all -- it comes from FFprobe -- so any run where FFprobe did not answer produces
+     * exactly this: real codec, real dimensions, real duration, `container = null`.
+     *
+     * **The twin was already tested and this one was not**, which is the argument for adding it.
+     * `FileCard` renders `probe.container?.label ?: "Unknown"` twice, once in the `AUDIO_ONLY`
+     * branch (`ConverterScreen.kt:660`) and once in the `VIDEO` branch (`:668`), and
+     * `an audio-only file nothing else could describe degrades one row at a time` drives only the
+     * first. Same expression, same fallback, one kind covered. That asymmetry is the same one
+     * `CLAUDE.md` records for including `ContainerCapabilities:94`.
+     *
+     * The other rows are asserted alongside so this is not a copy of the audio-only case: there,
+     * everything is unknown at once; here, one field is missing from a probe that is otherwise
+     * complete, and the rest must be unaffected by it.
+     */
+    @Test
+    fun `a video file whose container nothing identified says so and keeps its other rows`() {
+        setFileCard(input(probe = VIDEO_PROBE.copy(container = null)))
+
+        assertRow("Container", "Unknown")
+        assertRow("Video", "${VideoCodec.H264.label} · 1920×1080")
+        assertRow("Audio", AudioCodec.AAC.label)
+        assertRow("Length", "1:30")
+    }
+
     /**
      * The row is one node, not a label node beside a value node. A test matching on `"Container"`
      * alone would pass against either shape.