fix(editor): the TOC button keeps a gutter to stand on with line numbers off

[PathGao]

What this is

With "Line Numbers" off and the table of contents collapsed, the floating TOC
button sits on the text. Reported by @felhasznalonev in #810, second bullet, on
2.7.6 / macOS Apple Silicon. Not Closes: that issue carries six items and
this is one of them.

Mechanism

.toc-toggle-floating is positioned against the pane, not the editor — left: 8px, width: 28px, so it reaches 36px in. What it floats over is the
line-number gutter.

Measured on monaco-editor 0.55.1 with this app's creation options, Menlo 14px:

	glyph margin	line numbers	line decorations	contentLeft
Line Numbers on	0	42	26	68
Line Numbers off	0	0	26	26

EditorLayoutInfoComputer.computeLayout reserves that gutter on renderType !== Off, and 'off' is the only one of the five accepted values that
collapses it — 'on', 'relative', 'interval' and a render function all
keep it. A render function validates as Custom, so () => '' is "keep the
gutter, draw nothing in it": with line numbers off the layout is now identical
to line numbers on, minus the digits.

Scope

• Nothing narrows the kept gutter, so turning line numbers off no longer
  reclaims their width. I had a lineNumbersMinChars: 2 here that got it back
  to 17px, and took it out: the gutter is a multiple of the digit width while
  the button is a fixed 28px, so a narrowed gutter reintroduces the overlap
  below 8px of effective type (the default 14px at 50% zoom, the minimum 10px
  at 70%), and it leaves the button on the line-decoration strip that the full
  gutter clears. Matching "on" exactly has neither problem.
• tocSide: 'right' is untouched. The button mirrors to right: 8px, where
  what it covers is the vertical scrollbar rather than text, and nothing is
  reported about it.
• The other five items in Paragraph, icon, fonts, settings, sidebar, highlight #810 are separate; I have replied there with what
  each one looks like from the source.

Tests

One, in editorOptionWiring.test.ts. It asserts that "off" does not reach
Monaco as 'off' — that it arrives as a function returning '', that the
function is one module-level value rather than a closure per call (this runs
in an effect, and a fresh arrow would read as a changed option on every
updateOptions), and that the three modes which already reserved the gutter
are passed through untouched.

Reverted the change and kept the test: it fails.

Not pinned: the pixel widths. An earlier draft drove
EditorLayoutInfoComputer with the button's geometry read out of
MarkdownViewer.svelte, which needed two internals declared in
monacoInternals.d.ts and came to about 125 lines for a one-line fix. "Off
lays out like on" is the stronger invariant anyway — it survives the button
moving.

Verification

npm audit    # 0 vulnerabilities
npm run check    # 838 files, 0 errors, 0 warnings
npm test     # 1028 pass, 0 fail

cargo test not run — nothing under src-tauri/ changed.

Not verified: I have no screenshot of Markpad itself. The layout numbers come
from monaco-editor 0.55.1 driven directly with this app's creation options in
a browser, and the 36px from the stylesheet; that the editor pane starts at
the layout container's left edge is read off the CSS, not measured in the app.