Keep code literal and match its background to the theme

[scottjones]

Code containing Markdown characters was being styled as prose: default_line_height lost its underscores to italics, *value* inside a fenced block became emphasis, and the caret skipped visible characters as though they were hidden markup.

Treat inline code and triple-backtick fenced blocks as literal text, using the same parsing for highlighting and caret movement. Preserve underscores inside identifiers in ordinary prose. Emphasis and links can still surround inline code, while the code keeps its own styling and background. Escaped opening backticks stay literal, and closing fence lines no longer report hidden markers.

Code backgrounds now use the Omarchy theme's lighter_background. If that value is missing, invalid, or identical to the page background, Backend mixes the page color 6% toward the text color. This also works on pure black and white pages, and reloading a theme cannot retain the previous theme's panel color.

Validation

• Rebased onto current master (8f98892).
• QT_QPA_PLATFORMTHEME=generic bin/test: 21 passed, 0 failed on Qt 6.11.2.
• bin/build builds the application.
• Regression coverage includes escaped backticks, code inside bold/italic/link text, caret ranges on opening/interior/closing fence lines, unterminated fences and fence deletion, and theme color selection and fallback across reloads.

The inline-code portion overlaps with #48 and includes the same code-last rendering order needed to preserve nested code styling. This PR also handles underscores in prose, fenced blocks, and theme-derived code backgrounds.

The existing parser remains limited to single-backtick inline spans and the editor's triple-backtick fence convention. Fence backgrounds are painted per character, so their right edges follow line lengths and empty lines have no panel.

[omarchybot]

Reviewed both halves against the source and on a disposable VM. bin/test builds clean and all 16 tests pass on Qt 6.11.1. Nothing pushed — everything below is minor, and the two behaviour questions are yours to call.

The theme half holds up under the checks I had doubts about. m_themeLighterBackground.clear() at backend.cpp:599 is unconditional with no early return above it, and loadOmarchyTheme() is the only writer, so switching themes at runtime cannot inherit the previous theme's panel. .name() is never taken on an invalid QColor — lighter.isValid() guards the theme string and blend() always returns a valid fromRgbF colour — and that guard is load-bearing, since QColor("garbage").name() is #000000, which would have put a black panel on a light theme. The 6% mix measures #0f0f0f (lightness 15) on a pure black page and #f0f0f0 (240) on a pure white one, so the comment's claim about mixing beating lightening is right. All 22 stock themes ship lighter_background, and the only two where it equals the page — last-horizon and solitude — are exactly the fallback case; they land on #1a191a and #1b1e20.

Three small things:

• markdownhighlighter.cpp:118 stores the closing fence line as Prose — opening and interior lines get InsideFence, the closing one does not. That is what your own test asserts and it matches the fence-parity convention smartReturn() already uses in Main.qml, so it is defensible. The consequence is that backend.cpp:387 does not take its early return on that line: hiddenRangesAt() parses it as prose while highlightBlock() has painted the whole line with m_fenceFormat and hidden nothing. For a bare ``` no ranges come back and it is harmless, but on a line like ``` **x** the caret skips over four characters the user can see. Cheapest fix is to widen the guard to cover a fence line as well as an interior one.
• markdownhighlighter.cpp:225 — codeSpans() does not honour a backslash-escaped backtick, and it now decides whether emphasis is dropped rather than only where a panel is drawn. use \ in code like `x`renderscode` italic on master and renders it literally here. CommonMark agrees with your version for that particular line, so this is arguably a fix, but escaped backticks are a real divergence.
• The comment at markdownhighlighter.cpp:245 says markup that merely wraps a code span still applies, and the markup does — but the code span loses its panel while doing so. setFormat() replaces rather than merges, so the bold format applied over the content wipes the background: for **\file_name` only**` the whole run comes back with no background and visible backticks. This is not something you introduced — master does exactly the same — but the comment reads like a promise the rendering does not quite keep.

Unterminated fences behave correctly: every remaining block stays InsideFence to end of document, which is what CommonMark specifies, and the initial previousBlockState() == -1 is handled. Qt's incremental rehighlighting cascades properly when a fence is typed, deleted or split.

Heads up that PRs #5, #8, #15 and #17 all touch src/markdownhighlighter.{h,cpp} and tests/tst_omawrite.cpp; #15 adds another inline kind and will want the same code-span exclusion this PR introduces.