Feature request: add a project to a workspace from the Projects grid via a right-click menu #708

Short description

Add a project to a workspace from the Projects grid through a right-click menu (Add to workspace ▸ <workspace>), so the project side can reach the membership it already displays.

What problem does this solve?

A project's workspace is visible from the project side but only editable from the workspace side, and the workspace side is a multi-step detour:

• The Projects grid shows membership read-only. A card renders a 🗂 <name> pill when ProjectCard.workspace_name is set (packages/dashboard/src/components/Projects/ProjectsGrid.tsx:86-92), populated by workspace_name_by_identity (packages/dashboard/src-tauri/src/db.rs:5032-5043).
• Joining a workspace is only possible inside WorkspacesPanel: click + Add member (WorkspacesPanel.tsx:559-572), pick the project from a FilterSelect built by availableProjectsForWorkspace (WorkspacesPanel.tsx:519-531, workspace-staging.ts:138-146), optionally type a display name, click Add (handleStageAddMember, WorkspacesPanel.tsx:135-154). That only stages the change, so it still needs Save changes → applyWorkspaceChanges (WorkspacesPanel.tsx:196-218).
• So "put this project I am looking at into its workspace" costs: switch to Workspaces → find the target workspace → Add member → locate the project in a name-only dropdown (no search) → Add → Save.
• Right-clicking a project card does nothing in the app. The card is a plain <button> with only onClick (ProjectsGrid.tsx:70-73), and packages/dashboard/src contains no contextmenu handler at all, so the right-click menu you get is the webview's own (Reload / back / forward).

Proposed solution

Give project cards a context menu with a workspace submenu:

┌──────────────────────────┐
│ Open project             │
│ Add to workspace      ▸  │──┐
└──────────────────────────┘  │
   ┌───────────────────┐      │
   │ 🗂 magic-context  │◀─────┘
   │ 🗂 guizhan-account│
   └───────────────────┘

• Left-click keeps opening the project detail; only onContextMenu is added to the existing card control.
• The submenu lists the workspaces from the existing listWorkspaces() (src/lib/api.ts:94). The workspace the project already belongs to is marked rather than offered again.
• Picking a workspace commits immediately through the existing apply_workspace_changes, with no staging step — there is no workspace form on screen to save. Payload: addMembers: [{ project_path: card.identity, display_name: card.display_name, display_path: card.primary_path }], rename: null, removeMembers: [], setDisplayNames: [], and the workspace's current share_categories passed through unchanged.

No backend change is needed:

• apply_workspace_changes is differential. Share categories are written only when the normalized value differs from the stored one (packages/dashboard/src-tauri/src/workspaces.rs:743-744), and rename: None performs no rename (workspaces.rs:746-753), so an add-only payload cannot disturb the rest of the workspace.
• The client already has the membership data it needs: listWorkspaces() returns each workspace's members and share_categories (WorkspaceListItem, src/lib/types.ts:623-630).

Implementation notes:

• A project can belong to at most one workspace: CREATE UNIQUE INDEX idx_mc_workspace_member_unique ON mc_workspace_members(project_path) (crates/mc-store/src/lib.rs:716-717), and the backend rejects a project that already belongs elsewhere (workspaces.rs:692-701). The submenu should therefore be a single-choice list, and read Move to workspace ▸ (remove + add) when the project already has one.
• Home-directory projects cannot be members (workspaces.rs:653-657, is_user_home_workspace_member, workspaces.rs:841-860). The submenu should be hidden or disabled for them, or the backend error surfaced. The frontend cannot cheaply decide this itself — the home identity is dir:<md5(canonical_home)[..12]>, and the second check canonicalizes display_path, so this likely needs a flag on ProjectCard.
• FloatingLayer (src/components/ConfigEditor/FloatingLayer.tsx) already provides a portalled, viewport-aware layer with Esc and outside-click close, and is the natural container for such a menu (it would probably want to move to src/components/shared/).

Alternatives considered

• A hover button on the card. The card is a <button> element today (ProjectsGrid.tsx:70), so a nested button would be invalid HTML; that option costs a card rework (non-button container plus keyboard semantics) and is a bigger change than adding a context-menu handler.
• A workspace selector on the project detail page (ProjectDetail.tsx) — the same capability, one navigation deeper.
• A dedicated add_workspace_member command — unnecessary, because the differential apply_workspace_changes already accepts an add-only payload.

Area

Dashboard

Additional context

• Observed on the installed dashboard build 0.20.0 (/Applications/Magic Context Dashboard.app); UI references read from master.
• The project card and the project detail header already show which workspace a project is in; the gap is only the entry point from the project side.