docs(pics): add the v2.7.0 screenshots

[PathGao]

The v2.7.0 screenshots, in pics/2.7.0/ — the pics/<version>/ layout already used
by 2.6.2 and 2.6.3. Eleven images, all macOS, all against the same document so the
before/after pairs differ only in the thing they are about.

file	what it shows
preview-width-default.png	Preview width at its 880 px default (#346–#349). Also shows the settings rows renamed by #456 — Body font / Body font size / Code font / Code font size, which used to be two identical pairs of "Font" and "Font Size"
preview-width-wide.png	The same document at 1080 px. The blue bar left of Max width is #456's modified indicator, which replaced the · 640–1600 · Default 880 trailing text
export-html-before.png	Exported HTML before #411 — raw \(E = mc^2\), raw \begin{aligned}, unhighlighted code
export-html-after.png	The same export after #411 — KaTeX-rendered math, syntax highlighting
pdf-diagrams.png	A PDF export with Mermaid diagrams rendered in the light theme (#359), readable on white paper
editor-toolbar.png	The editor toolbar (#282) and its collapse control
editor-settings.png	Editor settings after #456 — one control column, units in a fixed slot
window-tag.png	Window tags (#280, #459): the chip, the popover, and the coloured line under the tab strip showing what the tag covers
floating-toc.png	The floating table of contents (#331), expanded
startup-js.png	#423 as a table — first-paint JS 4770 KB → 904 KB (−81 %); entering edit mode costs +3868 KB / 327 ms, once per window
split-view.png	Split view, editor left, preview right — the "example use case" shot for the README and markpad.dev

Three you asked for that are not here

The installer at large text (#428) is Windows-only. Worth flagging that the draft
release-note line for it is wrong: it reads "The Install button stays reachable at
large system text sizes", but #428's first paragraph says it is not about text
scaling — .content has min-height: auto inside overflow: hidden, so any
window shorter than its content clips the button with nothing to scroll it back.
Suggested wording: "The installer's content pane scrolls, so the Install button
stays reachable when the window is shorter than its content."

A PDF "before" cannot be produced: macOS PDF export did not work at all until
#287, so there is no earlier build to compare against. pdf-diagrams.png stands on
its own as "this is what it looks like now".

The collapsed floating ToC — the > chevron is visible in preview-width-default,
editor-settings and window-tag, so a dedicated shot for it seemed like noise.

Take whichever are useful and drop the rest; nothing else references these paths yet,
so removing a file here breaks nothing.