fix(markdown): preview local/relative images in .md files

The shared MarkdownText only renders absolute http(s) images (relative
links are disabled for chat security), so a local image referenced by a
previewed markdown file - ![alt](./img.png), an absolute /cwd/img.png,
or a reference definition - showed only its alt text.

Rewrite such destinations into absolute /sidebar/file media URLs (with
the GUI origin prefix) so MarkdownText accepts them and the host media
route serves the bytes (still restricted to the session cwd). The
rewriter is a dependency-free pure helper (markdown-images.ts) with a
focused unit spec; TextEditor passes the rewritten preview text to both
the plain MarkdownText and the mermaid-lazy preview.

src/client/TextEditor.tsx

@@ -22,6 +22,7 @@ import { EditorView as CodeMirrorView, keymap, lineNumbers } from '@codemirror/v
 import { defaultKeymap, history, historyKeymap } from '@codemirror/commands'
 import { IconCheckOutline16, MarkdownText } from '@deepseek-ai/dsh-client-ui-primitives'
 import { api, htmlUrl } from './api.ts'
+import { rewriteLocalImageUrls } from './markdown-images.ts'
 import { languageForPath } from './lang.ts'
 import { cmSurfaceTheme, CmThemeCompartment } from './cm-themes.ts'
 import { isDarkScheme, subscribeColorScheme } from './theme.ts'
@@ -252,6 +253,13 @@ export function TextEditor(props: FileViewerProps) {
   const html = viewerId === 'html'
   /** The markdown source the preview renders (draft wins over saved content). */
   const mdText = draft ?? content ?? ''
+  /** The preview source: `mdText` with local image destinations rewritten to
+   *  absolute media URLs (see {@link rewriteLocalImageUrls}); the raw
+   *  `mdText` stays untouched for selection/line lookup and for mermaid-block
+   *  detection, which are unaffected by image syntax. */
+  const previewText = markdown
+    ? rewriteLocalImageUrls(mdText, scope, path, window.location.origin)
+    : mdText
   /** md/mermaid block split for the preview (mermaid fences lift out). Split
    *  only in preview mode: edit-mode keystrokes must not re-scan the source. */
   const mdBlocks = useMemo(
@@ -392,8 +400,8 @@ export function TextEditor(props: FileViewerProps) {
               parse; cross-fence references/footnotes stay intact); files
               without one render exactly as before. */}
           {hasMermaid
-            ? <LazyMermaidMarkdown text={mdText} codeLabels={codeLabels} />
-            : <MarkdownText text={mdText} codeLabels={codeLabels} />}
+            ? <LazyMermaidMarkdown text={previewText} codeLabels={codeLabels} />
+            : <MarkdownText text={previewText} codeLabels={codeLabels} />}
         </div>
       )}
       {html && mode === 'preview' && (

src/client/markdown-images.ts

@@ -0,0 +1,89 @@
+/**
+ * Markdown-preview local-image resolution. The shared `MarkdownText` (from
+ * @deepseek-ai/dsh-client-ui-primitives) only renders absolute http(s) image
+ * URLs — relative links are disabled for chat security — so a local image in
+ * a previewed `.md` (`![alt](./img.png)`, an absolute `/cwd/img.png`, or a
+ * reference definition) would otherwise fall back to its alt text. This
+ * dependency-free helper rewrites those destinations into absolute
+ * `/sidebar/file` media URLs (prefixed with the GUI's own origin) so
+ * `MarkdownText` accepts them; the host media route then serves the bytes,
+ * still restricted to files under the session cwd.
+ */
+
+import type { SessionScope } from './api.ts'
+import { isAbsolutePath } from './paths.ts'
+
+/**
+ * True for a destination that is a remote URL — an absolute `scheme:` URL
+ * that is not a Windows drive path (`C:\...`). http/https/data/mailto etc.
+ * all match here and are handed back to `MarkdownText` untouched.
+ */
+function isRemoteUrl(dest: string): boolean {
+  return /^[a-z][a-z0-9+.-]*:/i.test(dest) && !/^[A-Za-z]:[\\/]/.test(dest)
+}
+
+/**
+ * Collapse `.`/`..` segments of an absolute local path, preserving its root
+ * (POSIX `/`), its Windows drive (`C:\`), or its UNC `\\server\share`
+ * prefix. The host's `requireAbsolute` (`path.resolve`) normalizes anyway,
+ * but producing a canonical path here keeps the `/sidebar/file` URL clean.
+ */
+function normalizeLocalPath(path: string): string {
+  const drive = /^([A-Za-z]:)[\\/]/.exec(path)?.[1]
+  const body = drive !== undefined ? path.slice(drive.length) : path
+  const parts = body.split(/[\\/]+/).filter((segment) => segment !== '' && segment !== '.')
+  const out: string[] = []
+  for (const part of parts) {
+    if (part === '..') { out.pop(); continue }
+    out.push(part)
+  }
+  if (drive !== undefined) return `${drive}\\${out.join('\\')}`
+  const separator = path.startsWith('\\') ? '\\' : '/'
+  const root = path.startsWith('/') ? '/' : path.startsWith('\\') ? '\\\\' : ''
+  return `${root}${out.join(separator)}`
+}
+
+/**
+ * Rewrite markdown image destinations that point at local files into
+ * absolute `/sidebar/file` media URLs. Relative destinations resolve against
+ * the opened file's directory (normalizing `.`/`..` segments); absolute
+ * local paths pass through. Remote (http/https/data/mailto) and `#`-anchor
+ * destinations are left untouched for `MarkdownText`. Reference-style images
+ * (`![x][id]` + `[id]: url`) are covered by rewriting their definition lines.
+ * @param text - The raw markdown source (inline + reference images).
+ * @param scope - The session scope (sessionId + cwd) for the media route.
+ * @param filePath - The absolute path of the opened `.md` file.
+ * @param origin - The GUI's own origin (`window.location.origin`); injected
+ * so the core rewrite stays pure and unit-testable.
+ * @returns The markdown with local image destinations rewritten in place.
+ */
+export function rewriteLocalImageUrls(
+  text: string,
+  scope: SessionScope,
+  filePath: string,
+  origin: string,
+): string {
+  const slash = Math.max(filePath.lastIndexOf('/'), filePath.lastIndexOf('\\'))
+  const directory = slash === -1 ? '/' : filePath.slice(0, slash + 1)
+  const mediaUrl = (candidate: string): string => {
+    // Mirrors api.ts fileUrl/mediaUrl for the /sidebar/file media route, made
+    // absolute so the shared MarkdownText http(s) allowlist accepts it.
+    const params = new URLSearchParams({ sessionId: scope.sessionId, path: candidate })
+    if (scope.cwd !== undefined && scope.cwd !== '') params.set('cwd', scope.cwd)
+    return `${origin}/sidebar/file?${params.toString()}`
+  }
+  const resolve = (dest: string): string => {
+    const trimmed = dest.trim()
+    if (trimmed === '' || trimmed.startsWith('#')) return dest
+    if (isRemoteUrl(trimmed)) return dest
+    const candidate = isAbsolutePath(trimmed) ? trimmed : directory + trimmed
+    return mediaUrl(normalizeLocalPath(candidate))
+  }
+  const inline = text.replace(/!\[([^\]]*)\]\(([^)\s]+)(?:\s+"[^"]*")?\)/g, (_match, alt, dest) => {
+    return `![${alt}](${resolve(dest)})`
+  })
+  // Reference-style images: rewrite their `[id]: <url>` definitions.
+  return inline.replace(/^(\s*\[[^\]]+\]:\s*)(<[^>]+>|[^\s]+)/gm, (_match, head, dest) => {
+    return `${head}${resolve(dest.replace(/^<|>$/g, ''))}`
+  })
+}