fix(frontend): make workflow-list failures visible and independent

WHY THIS MATTERS
Four failure paths in the rewritten WorkflowList were invisible or worse:

1. The initial load used Promise.all over the user's workflows AND the new
   add-on catalog, so a failing /workflow-addons endpoint blanked the whole
   workflows page - data that loaded fine before this feature existed.
2. importAddon() had no error handling at all: a failed import flipped the
   button back from "Importing..." with zero feedback, leaving the user to
   wonder whether they now own a copy (an unhandled promise rejection in
   the console was the only trace).
3. Bulk delete/import wrote "Some selected ... could not be ..." into
   loadError, which only renders inside the table's EMPTY state - invisible
   while rows exist, then leaking into the empty state much later.
4. openAddon() set the preview modal from a fetch with no staleness check:
   close the modal while the request is in flight and it pops back open;
   open A then B quickly and the modal can swap back to A's content.

WHAT IS THE PATTERN HERE?
Every fetch needs an answer to "what does the user see when this fails or
resolves late?" Promise.allSettled loads independent datasets independently;
a ref that records which add-on is currently open lets late responses be
recognized as stale and dropped; action errors get a dedicated, dismissible
banner that renders regardless of table contents.

HOW THE FIX WORKS
- Load: allSettled over (workflows, add-ons); each dataset renders or
  reports its own error (loadError for workflows, addonsError shown in the
  Add-ons tab).
- importAddon: in-flight guard (second click is a no-op), try/catch, and a
  visible "Could not import ..." banner on failure.
- Bulk actions report into the new actionError banner above the table.
- openAddon/closeAddon track the open add-on id in a ref and ignore stale
  responses.

Found by review of PR #309.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E4PXCdenNH5Mqhm5Sre9Zs