fix(kanban): filter dashboard board by selected tenant

[maciekczech]

Summary

• Fixes Kanban dashboard client-side filtering so the rendered board respects the selected tenant, not only assignee/search filters.
• Keeps the existing dashboard Select wiring through onValueChange/fallback handler.
• Adds regression coverage for both the Select handler wiring and tenant-aware client-side filtering.

Implementation note

plugins/kanban/dashboard/dist/index.js looks like a generated bundle, but for current Hermes dashboard plugins it is the committed runtime entry referenced by dashboard/manifest.json. This plugin currently has no checked-in src/, package.json, Vite/Rollup/esbuild config, or other source/build input under plugins/kanban/dashboard/; the tracked plugin files are only dist/index.js, dist/style.css, manifest.json, and plugin_api.py.

Why

The board API can fetch tenant-scoped data, but the dashboard also filters a cached board locally for search/assignee interactions. Without applying tenantFilter in filteredBoard and including it in the memo dependencies, tenant switches can leave stale/non-tenant cards visible until a full reload catches up.

Test Plan

• node --check plugins/kanban/dashboard/dist/index.js
• env -u HERMES_KANBAN_TASK -u HERMES_KANBAN_DB -u HERMES_KANBAN_WORKSPACE -u HERMES_KANBAN_WORKSPACES_ROOT scripts/run_tests.sh tests/plugins/test_kanban_dashboard_plugin.py -q
• Result: 45 passed, 4 warnings

[teknium1]

Merged via #21349 — both your commits cherry-picked onto current main with authorship preserved via rebase-merge. Live-verified the tenant dropdown actually filters rendered cards now (3 tasks × 2 tenants, all three selector states behave correctly). Thanks @maciekczech!