feat(web): parse markdown into the input model

[sbarczyk]

Web port of the markdown import, from Kotlin InputParser.kt and InputRemend.kt, cross-checked against iOS:

• InputRemend closes dangling delimiters before parsing (stack-based scan, link url mode, backslash escapes)
• InputParser walks the shared md4c AST into plain text plus formatting and block ranges: UTF-16 offsets come from the growing buffer, list depth from AST nesting, and blank-line runs are replayed from the source since md4c collapses them
• the wasm entry gains a permissiveAutolinks parameter so the input can disable bare-url autolinking like Android does; build.sh now links with em++ (required by current emscripten) and md4c.js is rebuilt
• round-trip tests on the real parser: canonical documents reproduce byte for byte, dangling forms normalize in one pass, a second pass changes nothing

Jest unit tests included. Stacked on #716.