feat: workspace categories (Identity → Category → Workspace grouping)

[careck]

Overview

Add a flat category structure to workspaces so the Open Workspace dialog groups entries as Identity → Category → Workspace instead of a flat list. This lets users organise related workspaces (e.g. Research, Personal, Work) under named categories per identity.

Categories are a local-only preference — stored in binding.json per workspace and in the identity's info.json — and are never synced through the swarm.

Key design decisions

• Storage: category: Option<String> added to WorkspaceBinding (binding.json); managed category list (Vec<String>) added to Identity (info.json). Both fields use #[serde(default)] for zero-migration backward compat.
• Logic in core: All category CRUD lives in IdentityManager in krillnotes-core. Tauri commands are thin wrappers.
• Managed categories: Renaming a category atomically updates all workspace bindings for that identity. Deleting nullifies bindings and prompts confirmation if workspaces are affected.
• UI: WorkspaceManagerDialog becomes a collapsible tree; inline category assignment per workspace row; "Manage Categories" panel per identity for rename/delete; optional category field on first workspace open/create.
• Identity filter dropdown in the current dialog is removed (redundant with tree grouping).

Spec

Full design: docs/superpowers/specs/2026-05-22-workspace-categories-design.md

Scope

• krillnotes-core: 5 new IdentityManager methods + extended WorkspaceBindingInfo
• src-tauri: 5 new thin Tauri command wrappers
• Frontend: WorkspaceManagerDialog restructure + i18n keys in all 7 locales
• No DB schema changes, no sync/swarm changes

[careck]

Implementation Plan

Full implementation plan: docs/superpowers/plans/2026-05-22-workspace-categories.md

11 tasks, fully sequenced:

1. Core data model — WorkspaceBinding.category + IdentityFile.workspace_categories + private read/write helpers
2. Core — list_workspace_categories + add_workspace_category
3. Core — rename_workspace_category + delete_workspace_category
4. Core — set_workspace_category
5. Tauri — WorkspaceBindingInfo.category + 5 command wrappers + registration
6. Tauri — WorkspaceEntry.category + list_workspace_files reads binding.json
7. TypeScript types + i18n — 7 locale files, workspaceCategories namespace
8. Frontend — tree skeleton (Identity → Category → Workspace, replace flat list)
9. Frontend — category assignment per workspace row (picker + inline new category)
10. Frontend — Manage Categories panel (rename/delete per identity group)
11. Frontend — optional category field in NewWorkspaceDialog