Give the editor a visual identity and a real levels control

The panel now uses the same colours the page is drawn with, on dark
chrome so the scan is the brightest thing on screen. Levels move from two
anonymous sliders to the scan's own histogram with draggable black and
white points and a tone strip, since getting them wrong is the one
mistake that only shows up on the tablet. A page rail replaces the
stepper and carries each page's slice count; export is pinned below the
scroll instead of below the fold; marker types read as prose.

docs/guide.md

@@ -44,14 +44,19 @@ off.
    one back.
 4. **Set the content rectangle.** Drag the blue edges so they hold the music and
    nothing else. This is the horizontal crop for every slice on the page.
-5. **Check black and white points.** These arrive proposed from the scan, like
-   the cuts do. If the paper still shows texture, pull *White point* down; if the
-   notes look grey rather than solid, pull *Black point* up. Getting this wrong
-   is the one mistake you cannot see until the bundle is on the tablet — grey ink
-   becomes half-transparent ink, and nothing downstream can rescue it.
+5. **Check black and white points.** Under *Page* is the scan's own histogram:
+   a hump of ink on the left, a hump of paper on the right, and two handles you
+   drag. Put the white handle at the foot of the paper hump and the black one at
+   the foot of the ink hump. The strip underneath shows the tone that results.
+   Getting this wrong is the one mistake you cannot see until the bundle is on
+   the tablet — grey ink becomes half-transparent ink, and nothing downstream
+   can rescue it. Arrow keys nudge the black point, Shift+arrows the white one.
 
-Page Up / Page Down move between pages. Levels carry over from the previous
-page, so a consistent scan only needs setting once.
+The page rail across the top of the panel has one chip per page, with the number
+of slices on it underneath. Click to go there, or use Page Up / Page Down. A page
+whose count is far off its neighbours' is usually a page where detection missed
+a system. Levels carry over from the previous page, so a consistent scan only
+needs setting once.
 
 ## Bar numbers
 
@@ -163,8 +168,8 @@ substitutes for it.
 | Detection found no systems, or one giant one | The score has no bracket joining the staves; add the cuts by hand. |
 | "The PDF has changed since these cuts were made" | The file was edited or replaced under an existing project. The cuts probably no longer line up — re-cut. |
 | Export says a title is required | Fill in *Song → Title*. |
-| Slices look grey and washed out | The white point is too high. |
-| Notes have holes in them | The black point is too high. |
+| Slices look grey and washed out | The white point is too high — drag it down onto the paper hump. |
+| Notes have holes in them | The black point is too high — drag it left, off the ink hump. |
 
 ## The command line