fix(workspace): use path.isAbsolute() for Windows path compatibility

[Lygggggg]

Closes #414

Summary

• Workspace panel shows empty on Windows because repoRoot.startsWith('/') rejects Windows absolute paths like C:\...
• Replace with path.isAbsolute() (already imported) which handles both Windows and Unix paths
• One-line fix, no new dependencies

Root Cause

GET /api/workspace/worktrees?repoRoot=C:\... returns 400 because the path validation assumes Unix-style absolute paths. Frontend catch { /* ignore */ } silently swallows the error, resulting in an empty workspace panel (no files, no changes, no git info).

Changes

1. packages/api/src/routes/workspace.ts:283 — startsWith('/') → isAbsolute()
2. packages/api/test/workspace-windows-path.test.js — regression test (7 cases)

Test Evidence

▶ workspace worktrees repoRoot validation (Windows compat)
  ✔ accepts Unix absolute path (1.186ms)
  ✔ accepts Windows absolute path with backslash (0.2177ms)
  ✔ accepts Windows absolute path with forward slash (1.3559ms)
  ✔ accepts UNC path (0.2021ms)
  ✔ rejects relative path (0.2148ms)
  ✔ rejects dot-relative path (0.1911ms)
  ✔ rejects parent-relative path (0.2409ms)
✔ 7 pass, 0 fail

Test plan

• Regression test covers Windows/Unix/UNC/relative paths
• On Windows: open a project, verify workspace panel shows files/changes/git
• On macOS/Linux: verify no regression (Unix paths still work)

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