Bulk add workspaces (multi-select folder picker)

[amanthanvi]

Problem

Adding many projects is currently tedious: the picker only allows selecting a single folder, so users must repeat "Add Workspace…" for every project.

Drag/drop can add multiple, but it loops adds without awaiting, which can cause focus/active-workspace bouncing and inconsistent results when many paths are dropped.

Proposal

Enable multi-select folder picking for workspaces and centralize bulk-add behavior so picker and drag/drop share the same pipeline.

UX

• "Add Workspace…" opens a directory picker that supports selecting multiple folders.
• After selection: add all valid, unique folders.
• Activation semantics: activate only the first newly-added workspace (predictable; avoids focus bouncing).
• Summary: show a single summary dialog only when something was skipped or failed (avoid noisy popups).

Edge cases

• Skip folders that are already present (path normalized; avoid duplicates).
• Skip non-directories (relevant for drag/drop, and defensive for picker).
• Partial failure: continue adding remaining selections; include failures in the summary (and/or Debug panel).

Implementation notes (suggested)

• Frontend
  • Add pickWorkspacePaths() helper (uses @tauri-apps/plugin-dialog open({ directory: true, multiple: true })).
  • Keep existing single-select helper for other flows (clone copies folder / git root pickers).
  • Add a bulk add function in useWorkspaces that:
    • normalizes + dedupes selected paths
    • skips existing workspace paths
    • adds sequentially via existing add_workspace IPC
    • activates only the first successful add
    • aggregates skipped/failed counts and shows one summary message when needed
  • Route drag/drop additions through the same bulk pipeline.
• Backend
  • No backend changes required (call existing add_workspace N times; should work in local + remote-backend modes).

Acceptance criteria

• Multi-select folder picker adds multiple workspaces in one operation.
• Only the first newly-added workspace becomes active.
• Existing workspaces are not duplicated; they are skipped.
• Drag/drop uses the same logic and does not bounce focus.
• A single summary dialog appears only when some selections were skipped/failed.
• Tests cover the new picker wrapper and bulk-add behavior.

[amanthanvi]

PR opened: #385