Files view: change details with before/after images (v2.4.0)

- the timeline's Δ glyph became a **Details** button, and it now appears on
  Update rows too (not just card-side changes); EN "Changed (card)" ->
  "Changed on card", RU "Изменено (картой)" -> "Изменено картой"
- the details modal shows the affected unit — the whole file, or the single
  record that changed — **before and after** the change as hex views (16 bytes
  per row, ASCII column, never-observed bytes as `--`), with the bytes that
  changed and the bytes the operation *first observed* highlighted in two
  different colours, plus the changed/new byte counts and the APDU reference;
  the old before/after byte table is gone
- `GET /api/sessions/<id>/files/image?key=…&change=…` returns those two views
  (segments with offsets) plus the change's metadata; the images are
  reconstructed from the current image by undoing the later changes
  (`FileTracker.change_images`), so nothing is duplicated per event — every
  value change is an event, hence the values are exact (a byte only first
  observed later can appear as known in both views)
- tracker: change events carry `observed_ranges` (bytes that were unknown
  before the operation) next to `ranges`; an update that only observes new
  bytes now records an event (so it can be inspected); record-file ranges are
  in-record byte offsets (the record number stays in its own field), which the
  previous "record number as position" could not express for a hex highlight
- tests: change-images reconstruction (before/after per event, later changes
  undone, records isolated, observation-only events, unknown gaps), the image
  endpoint (200/404/400), and Node tests for the hex-image helper (highlight
  classes, unknown gaps, ASCII, row limit) and the modal rendering
- v2.4.0; SW cache simtrace-analyser-v65

README.md

@@ -191,9 +191,20 @@ in the stored wire shape and are skipped with a note in the import report.
 The Trace tab's side panel has a view selector (*Map* / *Stats* / **Files**;
 a drop-down, since three tab captions no longer fit).  The Files view is a
 trace-wide timeline of file operations — every **Read**, **Update** and
-**Changed (card)** row shows the same file name as the Map view, and clicking
-a row jumps to the APDU in the trace.  A *Changed (card)* row carries a Δ
-button that opens the changed ranges with the previous and the new bytes.
+**Changed on card** row shows the same file name as the Map view, and clicking
+a row jumps to the APDU in the trace.  Rows that carry a recorded change (a
+card-side change, or an update that wrote or first observed bytes) also offer
+**Details**: a modal showing the affected unit — the whole file, or the single
+record that changed — **before and after** the change, as hex views (16 bytes
+per row with an ASCII column, never-observed bytes drawn as `--`) with the
+bytes that changed highlighted in one colour and the bytes the operation first
+observed in another, plus the changed/new byte counts and the APDU reference.
+
+The images come from `GET /api/sessions/<id>/files/image?key=…&change=…` and
+are reconstructed from the current image by undoing the later changes
+(`filetrack.FileTracker.change_images`) — nothing is duplicated per event.
+Every value change is recorded, so the values are exact; a byte that was only
+*first observed* after the selected change can appear as known in both views.
 
 The data comes from `simtrace_analyser/filetrack.py`
 (`GET /api/sessions/<id>/files`), a single-pass tracker over the decoded