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/api/files_test.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"bytes"
 	"net/http/httptest"
 	"net/url"
 	"os"
@@ -60,3 +61,44 @@ func TestFilesReadRanges(t *testing.T) {
 		}
 	}
 }
+
+// A write over the cap is refused whole: the file is not truncated first.
+func TestFilesWriteCap(t *testing.T) {
+	p := filepath.Join(t.TempDir(), "f.txt")
+	if err := os.WriteFile(p, []byte("keep me"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	s := &Server{fsLocal: files.Local{}}
+	body := make([]byte, 8<<20+1)
+	r := httptest.NewRequest("PUT", "/api/files/write?path="+url.QueryEscape(p), bytes.NewReader(body))
+	w := httptest.NewRecorder()
+	s.filesWrite(w, r)
+	if w.Code != 413 {
+		t.Fatalf("got %d", w.Code)
+	}
+	if b, _ := os.ReadFile(p); string(b) != "keep me" {
+		t.Fatalf("the file was touched: %q", b)
+	}
+	r = httptest.NewRequest("PUT", "/api/files/write?path="+url.QueryEscape(p), bytes.NewReader([]byte("new")))
+	w = httptest.NewRecorder()
+	s.filesWrite(w, r)
+	if b, _ := os.ReadFile(p); w.Code != 204 || string(b) != "new" {
+		t.Fatalf("a write within the cap: %d %q", w.Code, b)
+	}
+}
+
+// A link to nothing is a 404, not an empty 200.
+func TestFilesReadDanglingLink(t *testing.T) {
+	dir := t.TempDir()
+	l := filepath.Join(dir, "broken")
+	if err := os.Symlink("nowhere", l); err != nil {
+		t.Fatal(err)
+	}
+	s := &Server{fsLocal: files.Local{}}
+	r := httptest.NewRequest("GET", "/api/files/read?path="+url.QueryEscape(l), nil)
+	w := httptest.NewRecorder()
+	s.filesRead(w, r)
+	if w.Code != 404 {
+		t.Fatalf("got %d: %s", w.Code, w.Body.String())
+	}
+}