M6: Desktop endpoints/mounts + project multi-mount

[DeadWaveWave]

💡 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 Desktop Endpoint/Mount management and makes Projects/Spaces mount-aware:

• Add Endpoint/Mount management entry in Desktop settings and project UI
• Allow creating a Project with multiple mounts (local + remote), including remote-only
• Hide directory paths from Project list (show worker label/summary instead)
• Add Space target mount selection (v1: single target mount)
• Enable Agent running inside remote mounts (web nodes remain local-only for now)
• Route core runtime paths (filesystem ops / terminal spawn / agent launch) via mount-aware control surface

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

1. Context & Business Logic

• Users can register remote endpoints, create mounts (local or remote), and attach mounts to a Project.
• A Space chooses one target mount (v1). Terminal/Agent in that Space runs against the target mount.
• Project list no longer surfaces absolute directory paths.

2. State Ownership & Invariants

• Persistence owner: SQLite workspace state.
• Invariants:
  • A Space has at most one targetMountId (v1).
  • Terminal/Agent mount routing must be derived from targetMountId (not from UI-only state).
  • When target mount is remote, Agent launch must be routed through the selected endpoint.

3. Verification Plan & Regression Layer

• pnpm pre-commit (typecheck/lint/format/unit/contract + Playwright E2E) is green.
• Added E2E seeding to ensure mounts exist when UI queries mount data.

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✅ 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

TODO: attach screen recording in PR UI.