fix: surface a failed collections fetch instead of a false empty state (#6019)

[atomantic]

Summary

On /media/collections, refresh() swallowed a rejected listMediaCollections() into []. A transient 500, network timeout, or DB blip therefore rendered the "No collections yet" onboarding empty state and handed every image and video in the library to the synthetic "Unsorted" bucket — reading as "all my collections were deleted." There was no error banner and no retry path; once the request() toast faded, nothing on screen said a read had failed.

The fix applies the repo's sentinel convention (AGENTS.md, "Sentinel + validate to distinguish 'not set / failed' from 'present-but-empty / valid'"):

• collections is now null = never fetched / fetch failed, [] = the server genuinely has none.
• The collections leg of the Promise.all resolves to an array on success and an { error } envelope on failure, so the failure survives the join as data instead of flattening into an indistinguishable [].
• A failed read leaves collections alone — the sentinel on a first-load failure, the last good list on a later refresh — so the grid never blanks and the synthetic "Unsorted" bucket is skipped entirely rather than claiming the whole library is unfiled.
• A persistent Banner tone="error" names the failure, states that nothing was deleted, and offers a Retry that re-invokes refresh() (disabled while a load is in flight).
• The onboarding EmptyState now renders only on a genuine zero-collection response.
• The fetch passes { silent: true }, per the client convention that a caller owning its own error UI suppresses the shared request() toast.

handleCreate / handleDelete no longer assume an array: they extend or filter only a list that actually loaded, so a mutation can't clear the sentinel and resurrect the "everything is unsorted" state.

Test plan

Four new tests in client/src/pages/MediaCollections.test.jsx, each verified to fail against the pre-fix component (4 failed / 20 passed) and pass after:

• a rejected fetch renders the error banner + Retry, and renders neither "No collections yet", nor "Unsorted", nor the "no match" panel
• the fetch is issued with { silent: true } so the shared toast doesn't double-signal
• clicking Retry recovers the grid and clears the banner
• a later refresh failure (via the failed-delete recovery path) keeps the already-rendered collections on screen — scoped through the Alpha card with within() rather than indexing a shared-label match list

Full client suite: 833 files / 10238 tests passed, 1 skipped. (One unrelated LoomProductionPanel flake appeared on the first run; it passes in isolation on a clean tree and on a full-suite rerun.)

Closes #6019

https://claude.ai/code/session_01RA3pD5YM2dukQwbZ3pC6WA