fix(sync): tolerate existing WebDAV dirs after MKCOL auth errors

[codedogQBY]

Analysis

Issue #206 shows iPad WebDAV sync failing even though the same WebDAV account works on Android. The diagnostic log has a key pattern:

• PROPFIND / succeeds with 207, so credentials and the server URL are valid.
• The next MKCOL /readany returns 401, which aborts directory setup.

Some WebDAV servers are inconsistent around MKCOL for directories that already exist or are visible through the parent listing. Treating that response as a hard auth failure blocks sync even when the target collection is already present.

Changes

• When MKCOL fails or returns 401 / 403, re-check whether the collection already exists.
• Keep the existing fast PROPFIND Depth: 0 check, and add a parent-directory listing fallback for servers that refuse direct collection probes but expose the child in the parent listing.
• Continue only when the collection is proven to exist; genuinely missing or inaccessible directories still fail.
• Add regression coverage for the [Bug] webdav在pad401 #206 shape: direct /readany/ probe misses, MKCOL /readany/ returns 401, parent / listing includes /readany/, and ensureDirectory() proceeds.

Verification

• pnpm --filter @readany/core test -- webdav-client
• pnpm --filter @readany/core exec tsc --noEmit
• pnpm exec biome check packages/core/src/sync/webdav-client.ts packages/core/src/sync/webdav-client.test.ts
• git diff --check

Fixes #206