Collaborator dropdown error message is never cleared after being shown

[Alimedhat000]

Problem

client/src/hooks/use-collaborators.ts sets error in three places but only ever clears it at the start of a full refetch (setError(null) inside the docId effect, line 27):

• `Failed to add collaborator` (line 62, add failure)
• `Failed to remove collaborator` (line 49, remove failure)
• `Failed to load collaborators` (line 33, fetch failure)

Once any of these is set, the message persists indefinitely for as long as the hook is mounted with the same `docId`:

1. Successful retry doesn't clear it — `addCollaborator`'s success path (lines 56–59) never calls `setError(null)`, so the banner stays even though the collaborator was actually added.
2. Closing and reopening the dropdown doesn't clear it — state lives in the hook, which stays mounted with the document.
3. A successful `removeCollaborator` likewise never clears a previously-set error.

Impact

Users see a stale "Failed to add collaborator" alert next to a collaborator list that visibly does contain the just-added person. Contradictory UI; looks broken.

Fix

Clear the error at the natural transitions:

• reset `error` at the start of `addCollaborator` / `removeCollaborator` attempts (mirroring what the fetch effect already does), or
• clear it in each action's success path

Optionally also derive display from the most recent operation only.

Related: the `AddFailureShowsError` story (`collaborators-dropdown.stories.tsx`) exercises the error path but nothing asserts error clearing on recovery — worth adding once fixed.