feat: add open-in-file-manager action to project context menu

[Liu8Can]

💡 Change Scope

• Small Change: Fast feedback, localized UI/logic, low-risk.
• Large Change: New feature, cross-boundary logic, runtime-risk
  (persistence, IPC, lifecycle, recovery).

📝 What Does This PR Do?

Adds a third action to the project right-click menu so users can open
the project directly in the system file manager.

This change is intentionally minimal and reuses the existing
workspace.openPath path-opener flow instead of introducing a new main/
preload contract.

Behavior:

• Shows Open in Explorer on Windows, Open in Finder on macOS, and
  Open in File Manager on Linux.
• Resolves the project path from the current local mount first.
• Falls back to the legacy workspace.path only when mounts are still
  unavailable for older local data.
• Does not try to open a local file manager for remote-only projects;
  shows an in-app warning instead.

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🏗️ Large Change Spec (Required if "Large Change" is checked)

1. Context & Business Logic

N/A. This PR is scoped as a Small Change.

2. State Ownership & Invariants

N/A. This PR does not introduce new persisted or cross-boundary state
ownership.

3. Verification Plan & Regression Layer

N/A. See verification below.

---

✅ Delivery & Compliance Checklist

• My code passes the ultimate gatekeeper: pnpm pre-commit is
  completely green.
• I have signed the CLA if required (see CLA.md).
• I have included new tests to lock down the behavior (or explicitly
  stated why it's untestable).
• I have strictly adhered to the DEVELOPMENT.md architectural
  boundaries.
• I have attached a screenshot or screen recording (if this touches
  the UI).
• I have updated the documentation accordingly (if adding a feature
  or changing a contract).

📸 Screenshots / Visual Evidence

Please attach one screenshot of the project context menu showing:

• Manage locations…
• Open in Explorer / Open in Finder / Open in File Manager
• Remove project

Verification

• pnpm pre-commit
• Added unit coverage for:
  • local mount preferred over remote mount
  • legacy path fallback when no mounts exist yet
  • warning path for remote-only projects
• Added E2E coverage to verify the new context-menu action is visible
  from the sidebar project menu

[CLAassistant]

All committers have signed the CLA.

[Liu8Can]

I checked the failing CI after opening this PR.

What I changed in this PR is limited to the project sidebar context menu
and the new “open in file manager” action.

I re-ran the relevant checks locally:

• pnpm pre-commit passed
• the targeted sidebar E2E passed
• shard 2/3 (which contains workspace-canvas.sidebar- workspaces.spec.ts) passed locally

I also reproduced failures locally in unrelated M6 mounts tests:

• tests/e2e/m6.endpoints-mounts.integration.spec.ts
• tests/e2e/m6.endpoints-mounts.legacy- repair.local.integration.spec.ts

So from what I can tell, the current red CI is not caused by this
sidebar context-menu change.