Fix downloaded project ZIP using absolute paths (#147)

[cscheid]

Summary

Fixes #147. The hub-client Export ZIP feature produced archives whose entries were absolute (e.g. /cscheid/columns.qmd), because exportProjectAsZip used the stored index paths verbatim and those carry a leading slash. unzip refuses absolute paths and prints warning: stripped absolute path spec … for every entry.

Now entries are relative and nested under a single project-name folder, matching the download filename:

$ unzip -l Demo-Playground.zip
    Demo-Playground/cscheid/columns.qmd
    Demo-Playground/cscheid/crossrefs.qmd
    Demo-Playground/index.qmd

No warnings, one tidy Demo-Playground/ root.

Approach

The fix lives at the ZIP serialization boundary, not the storage convention (the leading-slash index paths are load-bearing across the sync client and index, so changing them is out of scope and risky):

• export-zip.ts — exportProjectAsZip(client, rootDir?) strips leading slashes so entries are relative, and nests every entry under a sanitized top-level folder when rootDir is given. The importer (parseProjectZip) already strips a common leading directory, so export/import round-trip cleanly.
• project-folder-name.ts (new, exported from quarto-sync-client) — derives a safe single path segment from the project name: Windows-hostile chars (< > : " / \ | ? *), path separators, and control chars collapse to hyphens; trailing dots/spaces are trimmed; empty falls back to project.
• automergeSync.ts — wrapper forwards an optional rootDir.
• ProjectTab.tsx — computes one slug via projectFolderName(project.description) and uses it for both the in-archive folder and the .zip download filename, so they can never drift.

Tests

• project-folder-name.test.ts — 9 cases (sanitization, fallback, leading/trailing trim).
• export-zip.test.ts — leading-slash stripping, rootDir nesting, no absolute entries, rootDir normalization, and an export→import round-trip.
• ProjectTab.test.tsx — jsdom wiring: same sanitized slug drives both onExportZip and the download filename.

All pass: quarto-sync-client 117, preview-runtime 74, hub-client 675 unit + 76 integration + 121 wasm. Production build (tsc -b && vite build) clean.

End-to-end verification

Ran the real exportProjectAsZip(client, 'Demo Playground') with absolute stored paths and inspected the bytes with the system unzip (the bug reporter's exact command) — entries are relative, nested under Demo-Playground/, and extraction emits no "stripped absolute path spec" warnings. The space in "Demo Playground" is sanitized to Demo-Playground, matching the download filename.

Not run this session (noted as optional follow-ups in the plan): a live-browser hub session and a Playwright export e2e (there is no export e2e today).

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