Files: a sweep of the edges

Found by walking every handler and the UI with odd inputs, then a battery of requests
against the live hub. Fixed:
- A save over the 8 MiB cap was cut short in place: the file was truncated, then the
  first 8 MiB written, and the caller told 204. Now the body is read whole first and a
  body over the cap is a 413 with the file untouched.
- A link to a file showed the link's own time; now the file's, so the row reads right
  and the preview key follows the file.
- Reading a link to nothing answered 200 with an empty body (the open failed after the
  headers in root mode); now a 404 that names the target.
- A play request with an unknown codec or size was answered by inspecting the file
  first, so an audio-only file said "no video stream" instead of what was wrong; the
  choice is checked before any work, and is a 400.
- A stream that ended in failure was handed back to the next identical request for ten
  minutes; a failed one is now dropped and a fresh one made.
- A player that left while its stream was still finding a first segment made the loop
  try the next recipe; it stops. A stream closed mid-start is not started again.
- Ending a root-mode ffmpeg could wait forever: the daemon cannot signal root's
  processes, so the wait is bounded and the helper's own grace does the rest.
- A page going away (a phone closing the tab) releases its stream by a beacon, rather
  than ten minutes later by the sweeper.
- The path box resolves like a shell — relative to the folder shown, . and .. applied,
  trailing slashes dropped — in the tab and in the Move picker, with a test. A typed path
  that is a file opens it; a remembered folder that is gone falls back to / once.
- A dropped folder arrived as a nameless empty file; it is refused with a word. The
  upload toast counts what was actually uploaded.
- The viewer names a device, pipe or socket, and a dangling link, instead of calling
  them binary files.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/media/encode_test.go

@@ -21,7 +21,7 @@ func TestFit(t *testing.T) {
 		{"original", src("hevc", 3840, 2160, 30), "h264", 3840, 2160, 20000},
 		{"720p", src("h264", 2292, 960, 60), "h264", 1718, 720, 5000},  // odd width made even
 		{"1080p", src("h264", 1280, 720, 30), "h264", 1280, 720, 4000}, // never upscaled
-		{"480p", src("h264", 1920, 1080, 30), "hevc", 852, 480, 1200}, // 853.3 rounds down, then even
+		{"480p", src("h264", 1920, 1080, 30), "hevc", 852, 480, 1200},  // 853.3 rounds down, then even
 		{"original", src("av1", 640, 360, 24), "av1", 640, 360, 1200},
 	} {
 		w, h, k := fit(q(c.q), c.in, c.codec)
@@ -117,3 +117,12 @@ func TestParseProgress(t *testing.T) {
 		t.Fatalf("got %v %v", s, sp)
 	}
 }
+
+func TestValidChoice(t *testing.T) {
+	if err := ValidChoice("h264", "720p"); err != nil {
+		t.Fatal(err)
+	}
+	if ValidChoice("vp8", "720p") == nil || ValidChoice("h264", "9p") == nil || ValidChoice("", "") == nil {
+		t.Fatal("bad choices must be refused")
+	}
+}