web: undo/redo, a clean preview, and a histogram that stays put

Four things the studio owed the visitor:
- UNDO/REDO in the header, so a look can be taken back and put back without
  reloading the photo; a fresh edit clears the redo trail.
- Opening a saved frame, or picking a look out of its history, now drops the
  stale preview buffer instead of leaving the previous render on the stage.
- The picked history look is marked in the accent, so it is plain which look
  the photo is wearing.
- The histogram is re-clamped against the photo box on resize, so opening a
  chip column no longer pushes the overlay past the edge of the canvas.

The 'NEW SAVES: FILM STRIP' chip goes: a save already lands in the strip.

docker/frontend/src/styles/app.css

@@ -103,6 +103,8 @@
 /* In a column the chips stack: one chip per row, full width. */
 .col .chip-row { flex-direction: column; align-items: stretch; }
 .col .chip { justify-content: space-between; text-align: left; }
+/* UNDO/REDO are one glyph each, so the pair reads as a single control. */
+.header .btn.icon { padding: 4px 9px; font-size: 15px; line-height: 1.2; }
 .wm-extra { border-right-color: var(--accent-line); }
 .chip {
   border: 1px solid var(--border);