Log files: one file per entry, format detection

- a path is one file (wildcards refused for now); the name defaults to
  the file name
- format "auto" (the default): detected from the last 50 lines of the
  file, the structured format followed by at least 80 % of them (Traefik
  access JSON, JSON lines, CLF, logfmt), else plain text; done again when
  the format changes, shown in the status ("Format detected: …"); any
  other format forces it
- POST /api/files/detect {path}: found / permission / empty, detected
  format and last line, shown under the row as soon as the path is typed
  (POST so that read-only accounts cannot read files through it)
- README (EN/FR) updated

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

web/style.css

@@ -773,7 +773,12 @@ input.switch:focus-visible { outline: 2px solid var(--accent); outline-offset: 2
 .file-row select.field { height: 28px; }
 .file-row .icon-btn { width: 26px; height: 26px; border-radius: 6px; }
 .file-row .icon-btn svg { width: 15px; height: 15px; }
-.file-row .fr-regex, .file-row .fr-status { grid-column: 2 / -1; }
+.file-row .fr-regex, .file-row .fr-status, .file-row .fr-sample { grid-column: 2 / -1; }
+.file-row .fr-sample {
+  margin: 0; font: 11.5px var(--mono); color: var(--muted);
+  white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
+}
+.file-row .fr-sample[hidden] { display: none; }
 .file-row .fr-regex[hidden] { display: none; }
 .file-row .fr-status { margin: 0 !important; font-size: 12px; }
 .file-toolbar { margin-top: 10px; }