fix(preview): resolve relative images of imported documents correctly

[harsh-thakkar7]

Summary

Opening an imported document (.html, .pdf, .docx) rendered its relative
images against the wrong folder, so they were broken in the preview.

Root cause

openFile()'s conversion branch was the only refreshPreview call site that did
not pass a base directory:

const newTab = { ...makeTab(id, label, markdown, null, null), dirty: true };
setTabs((prev) => [...prev, newTab]);
setActiveTabId(id);
refreshPreview(markdown);          // <- no baseDirOverride

The fallback inside refreshPreview then used activeTab.filePath. But the
closure belongs to the render before the tab switch, and setActiveTabId
has not re-rendered yet — so it read the previously active tab's path.

Every other call site passes the path explicitly:

call site	passes base dir
handleContentChange	yes
openFile (already-open tab)	yes
openFile (plain read)	yes
openTextAsTab	yes
openFile (conversion)	no

Why it cannot just use filePath

A converted tab is created with filePath: null, on purpose — so Save asks for a
destination instead of overwriting the source .html. But its image paths are
relative to where that file lived.

The backend resolves them with fix_image_paths():

abs_path = os.path.abspath(os.path.join(base_path, src))

So the wrong base_dir produces concretely wrong file:// URLs. Verified
end-to-end from the real HTML conversion:

>>> md = convert_html_to_markdown('<img src="chart.png">')
'![chart](chart.png)'                      # relative path survives conversion
>>> fix_image_paths(md, "/work/notes")
'![chart](file:///work/notes/chart.png)'   # correct
>>> fix_image_paths(md, "/elsewhere")
'![chart](file:///elsewhere/chart.png)'    # broken: previous tab's folder

If the previously active tab was untitled, filePath is null, so base_dir
was undefined and the relative src resolved against the app origin inside
the sandboxed preview iframe — also broken.

Repro: keep /elsewhere/old.md active, then open
/work/notes/report.html which references chart.png sitting beside it. Every
relative image renders broken.

Changes

frontend/src/lib/preview-base-dir.mjs (new) — parentDirOf() and
resolvePreviewBaseDir(), holding the rule in a plain ESM module so the Node
regression suite can exercise the exact code the app runs (same pattern as
http-timeout.mjs).

frontend/src/hooks/useEditor.ts

• Tab gains previewBaseDir, defaulting to the parent of filePath. For a
  converted tab it is set to the folder the source file came from.
• refreshPreview's fallback consults previewBaseDir before filePath, so the
  correct directory survives later re-renders too — dark-mode toggles,
  font-size changes and every keystroke all re-render through the fallback.
• The conversion branch passes the base dir explicitly for the first paint,
  before the tab switch has taken effect.

Behaviour for plain Markdown tabs is unchanged: previewBaseDir is just
parentDirOf(filePath).

I deliberately left the DOCX/PDF export paths (useFileIO.ts,
useActions.ts) alone. They derive base_dir the same way and so have the same
latent issue for converted documents, but that is a separate behaviour change and
this PR stays on the preview.

Testing

frontend/tests/preview-base-dir.test.mjs, 9 cases exercising the real helper:

• parentDirOf for POSIX and Windows separators, root-level, and no-directory
• an explicit override wins
• a plain Markdown tab resolves against its own folder
• a converted document resolves against the folder it was imported from
• previewBaseDir wins when both are present
• an untitled tab, null, and missing arguments return undefined
• the result is a directory ending in a separator, never a file path

Verified 3 fail when the previewBaseDir branch is removed from
resolvePreviewBaseDir.

Verification

• npm test — 32 passed (was 23 on main; this PR adds 9)
• npm run lint — 0 errors, 0 warnings on the changed file
• npm run build — clean
• backend pytest — 195 passed, unchanged

[petertzy]

Approved after follow-up fixes. The original change correctly preserves the source directory for imported HTML/PDF/DOCX documents and passes it explicitly during the initial preview render. I also fixed two related lifecycle cases: switching to a converted tab after closing another tab, and saving a converted document to a new path. These cases now preserve or update the preview base directory correctly. Frontend tests pass (48/48) and lint passes.