fix(desktop): use config-relative glob so desktop stories load on Windows

[bytelazy]

Summary

On Windows, Storybook run from apps/desktop never indexes the desktop story
files: index.json reports 53 entries (only the packages/ui stories) and
none of the desktop stories appear in the sidebar. The desktop stories glob was
built with resolve(REPO_ROOT, 'apps/desktop/stories/**/*.stories.@(ts|tsx)'),
which produces a backslash absolute path on Windows. Glob matchers treat
backslashes as escape characters, so the pattern matches nothing and the entry
is silently dropped. The neighboring packages/ui entry is a forward-slash,
config-relative glob — which is why exactly the UI stories kept working, and
why CI (Linux) never caught it: a resolved absolute POSIX path still globs
correctly.

Switch the desktop entry to the same config-relative, forward-slash form
('../stories/**/*.stories.@(ts|tsx)') so the glob matches on all platforms.

Fixes #4516

Verification

Verified the glob behavior directly with fast-glob (the matcher Storybook
uses) against a clean checkout:

• The new config-relative glob ../stories/**/*.stories.@(ts|tsx) resolves
  from apps/desktop/.storybook/ and matches all 15 desktop story files
  (including the settings/ and subagent-sessions entries).
• The old entry as an absolute POSIX path matches the same 15 — which is why
  CI (Linux) never caught the bug.
• The old entry rendered with Windows-style backslashes (E:\...\stories\**)
  matches 0 files, reproducing the reported Windows failure.

Not run: a full Storybook launch on Windows (no Windows machine here), and the
repo's lint/typecheck/test suites (a full npm ci could not complete from this
network). The glob-level reproduction above demonstrates both the bug and the
fix; a reviewer on Windows can confirm end-to-end with npm run storybook in
apps/desktop and check http://localhost:6006/index.json (expected: 251
entries, all desktop stories present).

AI use

• Generative tooling made a substantive contribution

Tool(s) and scope: Claude Code — identified the fix, edited
apps/desktop/.storybook/main.ts, and authored the commit. The commit carries
a Generated-by: Claude (Claude Code) trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above

[Astro-Han]

Same one-line fix as #4517, which was opened a few hours earlier for #4516 and is green, so closing this one in favour of it. Thanks for the write-up; the backslash explanation matches.