feat(files): render markdown in the File Viewer, with Lines/Wrap toggles

Clicking a .md in the Files panel showed wrapped source with an Edit
pencil and no way to see it rendered, although marked + DOMPurify were
already on the page for the Response Viewer. The viewer now renders
.md/.markdown through that same pipeline (one parser, one click
delegate) with an MD pill back to source, and the plain-text view gains
Lines (CSS-counter gutter) and Wrap toggles. All three persist per device
in their own localStorage keys.

- Relative images are rebased onto the workspace-confined file-raw route
  under the document's directory, built inside a <template> so no fetch
  fires before the rewrite; a failed load degrades to alt text. Relative
  links become a.rv-path so the existing delegate opens them in the
  viewer; fragment and http(s) links are untouched.
- The rendered container carries data-i18n-skip so the translator does
  not rewrite the document's prose.
- Markdown fetches the route's 10000-line ceiling; other text keeps 500.
- avif renders inline (file-content image set, file-raw MIME map), and
  avif/ico printed paths open the viewer instead of tailing bytes. .md
  deliberately stays with the tail viewer for printed paths.

docs/architecture-invariants.md

@@ -384,6 +384,19 @@ The general rule: **any new endpoint that turns a caller-supplied `sessionId` in
 
 Tests: `test/file-editing-policy.test.ts` (pure policy), `test/routes/file-write-routes.test.ts` (deliberately **unmocked fs** against a real temp workspace — symlink/TOCTOU/mode behavior must be exercised for real).
 
+### File Viewer text view: rendered markdown and text toggles
+
+**Rendered markdown + Lines/Wrap** (`_renderFilePreviewText()` and its helpers in `panels-ui.js`, buttons in the `.file-preview-actions` row): a `.md`/`.markdown` opened in the File Viewer renders as a document by default, with an `MD` pill back to source; the plain-text view has `Lines` (a CSS-counter gutter) and `Wrap` toggles. Codeman already had `marked` + DOMPurify behind `_renderMarkdown()` for the Response Viewer, so the viewer reuses that and the codebase keeps ONE markdown pipeline.
+
+- ⚠️ **One pipeline, one delegate.** The viewer calls `_renderMarkdown()` (marked + the `sanitize-html.js` allowlist) and binds `_bindResponseViewerInteractions()` on `#filePreviewBody` (container-bound and idempotent, so once per page) for the code-block copy buttons and `a.rv-path` opening. Never a second parser, never a second click handler for the same markup.
+- ⚠️ **Build inside a `<template>`, then rebase.** A detached div whose `innerHTML` is set starts fetching every `<img src>` at once, so the document's relative image paths would hit the server as `/docs/img.png` 404s before being rewritten. `_rebaseFilePreviewMarkdownRefs()` runs on the template content: relative images go to the workspace-confined `file-raw` under the document's directory (the server refuses escapes, so `..` is forwarded as-is), and one `error` handler per image degrades it to alt text, which covers a remote image the page CSP blocks, a 404 for a document outside the workspace, and an SVG that `file-raw` serves as a download. Never widen a route for this. Relative links become `a.rv-path` with `data-path` and lose the `target`/`rel` that `_renderMarkdown` gives every link, which would otherwise open `<origin>/docs/x.md` in a new tab; fragment and http(s) links are untouched.
+- ⚠️ **`data-i18n-skip` on the container.** The translator's MutationObserver translates inserted headings and paragraphs, and the `.file-preview-content` entry in its skip list matches nothing (no element has that class), so the attribute is what keeps a Chinese UI from rewriting a README.
+- ⚠️ **Toggles are per-device, in their own localStorage keys** (`codeman:filePreviewMdRendered` / `LineNumbers` / `Wrap`), for the same reason as the Files panel's show-hidden toggle: the app-settings object is rebuilt from the settings modal on every save, and they are display state, not synced settings (`SettingsUpdateSchema` is `.strict()`). MD re-renders from the kept source (`filePreviewContent`, which is also what Copy copies) without a refetch; Lines/Wrap are class flips on the one `<pre>`, whose rules are scoped `.file-preview-body > pre.file-preview-text` so they never leak into the document's own code blocks. Lines are one inline `<span class="fp-line">` per line joined by real newlines, the counter in `::before` with `user-select: none`, so select and copy return the exact text.
+- ⚠️ **Caps.** Markdown fetches `lines=10000` (the route's `MAX_LINES_LIMIT`), because a rendered document cut at 500 lines reads as the whole document; other text keeps 500, which is what stops a huge log locking the tab in one `<pre>`. The attachment (out-of-workspace) branch keeps its 512 KB Range read and skips the 500-line clip for markdown. Edit mode is unchanged and still re-fetches `edit=1`; the three toggles hide while editing and for images, media and PDFs.
+- ⚠️ **`md` is NOT in `FILE_PREVIEW_EXTENSIONS`.** A `.md` path printed in the terminal or chat still opens the tail viewer (see File-path links above: in-workspace text keeps live follow, which is what Ralph's `fix_plan.md` needs); the rendered view is reached from the Files panel. `avif` and `ico` were added there (a printed `favicon.ico` used to tail binary noise), with `avif` also in file-content's image set and file-raw's MIME map; out-of-workspace avif/ico stay unregistrable, like svg/bmp.
+
+Tests: `test/file-preview-markdown.test.ts` (jsdom-in-vm, pins every rule above), `test/routes/file-routes.test.ts` (avif).
+
 ### Clone a repository as a case
 
 **Clone Repo tab** (issue #236, proposed by @DodgyBadger): `POST /api/cases/clone` clones a public repository into the caller's case space and registers it as a normal local case; `POST /api/cases/clone-preflight` answers "can this be cloned anonymously, and what refs does it have?" while the user is still typing. Core in `src/git-clone.ts`, split into a PURE half (URL parse, argv/env, `ls-remote` parse, stderr classification) and a thin IO half (`probeGitRemote`, `cloneRepository`).

docs/wiki/Working-With-Files.md

@@ -13,7 +13,8 @@ It renders what it can:
 
 | Kind                     | Behaviour                                                                 |
 | ------------------------ | ------------------------------------------------------------------------- |
-| Text and code            | Syntax-aware preview. Long files are truncated in plain preview.           |
+| Text and code            | Plain preview with Lines (line numbers) and Wrap toggles in the header. Long files are truncated in plain preview. |
+| Markdown                 | Rendered by default: headings, tables, code blocks with copy buttons, images and links relative to the file. The MD pill in the header flips to source. |
 | Images                   | Inline.                                                                    |
 | Audio and video          | Inline with a working scrub bar, because range requests are supported.     |
 | PDF and Office documents | Converted for preview when a converter is available.                       |