* feat(chat): live markdown source-mode highlighting in composer
Highlight markdown syntax, fenced code blocks, and mention-style tokens
directly in the chat input via the existing transparent-textarea overlay
(color/decoration/background only, so caret alignment is preserved).
- Markdown source-mode: inline/fenced code, links, headings, blockquotes,
list markers, with dimmed syntax punctuation
- Per-language syntax highlighting inside fenced blocks, reusing the editor's
CodeMirror language resolver + Lezer (bash/js/ts/json/html/css/python/md);
highlighted blocks use a neutral base, plain fences keep the code color
- Token highlighting on match: @file, @agent, /command, /skill, #snippet
- Auto-pairing: wrap selection with markers, triple-backtick expands to a
fenced block; paste a URL over a selection to form a markdown link
- Add md/markdown to the shared code-block language resolver
* fix(chat): address composer highlight review
- Tilde (~~~) fenced blocks now get per-language syntax highlighting
- Share fence open/close detection between tokenizeMarkdown and
highlightFencedCode so they agree on boundaries (fence length + format),
fixing range bleed with 4-backtick fences and ```lang lines inside blocks
- Replace buildHighlightParts O(segments x ranges) scan with a sweep-line
over an active set (verified equivalent vs the prior algorithm across 30k
randomized cases, including overlaps and explicit class/priority)
- Cap per-block Lezer parsing at 20k chars; oversized blocks keep the neutral
code base without per-token coloring
