feat(workspaces): import a VS Code .code-workspace as a workspace

[danielss-dev]

Summary

Closes the second Workspaces Phase 3 item: a VS Code multi-root .code-workspace file imports as a named Strand workspace.

• Surfaces: palette "Import .code-workspace..." (imports, opens the workspace, toasts added/skipped) and an "Import .code-workspace..." row in the manager dialog (imports and selects it for curation; skipped folders surface in the message slot).
• Parsing is pure, unit-tested TS (lib/codeWorkspace.ts): JSONC-tolerant (string-aware comment + trailing-comma stripping), local path entries only (remote uri ignored), relative paths joined against the file directory without canonicalizing — each folder is validated through repoOpen and the canonical meta.path is what gets stored (the Windows re-spelling rule).
• File reading is a new deliberately narrow IPC command workspace_file_read: the name must end .code-workspace and the file be <= 1 MB, so the webview gains no generic file reader out of this.
• Non-repo folders are skipped and reported; the import only errors when no folder resolves to a repository.

Test plan

• cargo test -p strand-tauri — 12/12 (incl. new workspace_file_read gate test)
• cargo clippy -p strand-core -p strand-tauri -- -D warnings — clean
• pnpm --filter ./ui exec tsc --noEmit — clean
• pnpm --filter ./ui test — 171/171 (+13 codeWorkspace tests)
• pnpm --filter ./ui exec vite build — green
• End-to-end against the running Tauri app over WebView2 CDP: a JSONC fixture with two real repos + a non-repo folder + a remote uri imported as "acme" (2 added, 1 skipped, uri ignored) and opened with members tabbed; an all-non-repo file threw without creating anything; the backend refused a non-.code-workspace path.

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