Port of upstream PR marktext#5004 (follow-up to marktext#4231, related to marktext#2002).

Adds a default-on 'Inline math' preference under Preferences ->
Markdown -> Extensions. When disabled, single-dollar expressions stay
literal text — currency-heavy prose like $13B is no longer swallowed
into KaTeX — while double-dollar display math keeps rendering. The
option threads through every parse/render entry point: live editing,
markdown import, clipboard HTML, TOC text, and static HTML/export.

Deviations from the upstream PR: the schema.json hunk is re-applied on
top of this fork's separator fixes (e197fb8), and the ja / zh-CN /
zh-TW locale strings are translated instead of the English
placeholders.

Round-trip serialization (marktext#4934 concern) verified: muya spec suites
including roundTrip.spec pass, and the PR's own test pins
getMarkdown() identity with inlineMath disabled.

…5446) (#5450)

`math` was a half-finished master switch: it gated only the marked path
(import, HTML export, clipboard), the live editor ignored it entirely,
and MarkText never set it — no schema entry, no UI, no setOptions call.
So the option that was supposed to control dollar math could not turn it
off, and where it did apply it made the editor disagree with its export.

It is now `texMathDollars`, named after the pandoc extension it
implements, with the live-editor gate it never had and a preference
under Preferences → Markdown → Extensions beside the other pandoc-named
toggles. Default on, as in pandoc's own `markdown` dialect.

It covers `$…$` and `$$…$$` together, because that is the granularity
pandoc gives the extension — the question #4231 stalled on, answered by
adopting an existing definition rather than inventing one. This is also
the switch #2002, #4231 and #5243 asked for, the last of them patching
app.asar to get it.

With the option off the syntax is neither read nor offered: the inline
and begin rules skip it, `$$` + Enter stays a paragraph, and the menus
that list blocks — quick insert, the front menu's Turn Into, and the
desktop Paragraph menu — drop Math Block. `canInsertLabel` is the single
rule behind that, consulted by those menus and by the builder every
insertion path funnels through, so the ⌥⌘M shortcut cannot slip past a
list it never reads.

Task 2 of #5446.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

…arktext#5446) (marktext#5450)

`math` was a half-finished master switch: it gated only the marked path
(import, HTML export, clipboard), the live editor ignored it entirely,
and MarkText never set it — no schema entry, no UI, no setOptions call.
So the option that was supposed to control dollar math could not turn it
off, and where it did apply it made the editor disagree with its export.

It is now `texMathDollars`, named after the pandoc extension it
implements, with the live-editor gate it never had and a preference
under Preferences → Markdown → Extensions beside the other pandoc-named
toggles. Default on, as in pandoc's own `markdown` dialect.

It covers `$…$` and `$$…$$` together, because that is the granularity
pandoc gives the extension — the question marktext#4231 stalled on, answered by
adopting an existing definition rather than inventing one. This is also
the switch marktext#2002, marktext#4231 and marktext#5243 asked for, the last of them patching
app.asar to get it.

With the option off the syntax is neither read nor offered: the inline
and begin rules skip it, `$$` + Enter stays a paragraph, and the menus
that list blocks — quick insert, the front menu's Turn Into, and the
desktop Paragraph menu — drop Math Block. `canInsertLabel` is the single
rule behind that, consulted by those menus and by the builder every
insertion path funnels through, so the ⌥⌘M shortcut cannot slip past a
list it never reads.

Task 2 of marktext#5446.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
(cherry picked from commit 3e9e91c)