fix(desktop,relay): restore multi-channel workflows list

[Trevongit]

Summary

Desktop Workflows showed No workflows yet after successful saves even when CLI buzz workflows list --channel <uuid> returned rows.

Root cause: get_channels_workflows packed all member channel IDs into one multi-value #h filter. Relay extract_channel_id_from_filter collapsed multi-#h to the lexicographically first channel UUID and pushed that as SQL channel_id, so the query often returned only an empty channel (e.g. #general).

Fix (defense in depth):

1. Desktop — one single-value #h filter per channel in a single POST /query (NIP-01 OR; each filter is SQL-pushable).
2. Relay — multi-value #h returns None from the singular extractor (same as bridge/count siblings), so multi-#h widens correctly instead of pinning one channel.
3. UX — avoid flashing the empty state while channel membership is still loading.

Live Groundfeed repro before this change:

• #h: [welcome] → workflows present
• #h: [welcome, skills, general] → 0
• two single-#h filters OR → full set

Related work on block/buzz

Others hit the same bug; these open PRs address the relay half (not merged as of this PR):

• fix(relay): stop collapsing multi-value #h filters to a single channel #2452 — stop collapsing multi-value #h
• fix(4381): return None for multi-value #h in singular req extractor #4393 — same extractor fix for POST /query returns events from only one channel when a filter has multiple #h values #4381
• fix(relay): preserve multi-channel workflow queries #2350 — preserve multi-channel workflow queries

This PR is complementary: it keeps the Desktop list correct even against unfixed relays, and still hardens the relay path.

Test plan

• cargo test -p buzz-relay multi_h_filter_does_not_collapse / single_h_filter_extracts pass
• cargo test --manifest-path desktop/src-tauri/Cargo.toml workflows (11 unit tests) pass
• Rebuild Desktop from this branch; open Workflows → list should show workflows across member channels (not empty when CLI list is non-empty)
• Optional: against Groundfeed, confirm multi-channel overview after Desktop rebuild

[Chkhikvadze]

Independent production reproduction and validation:

• The deployed relay currently returns an empty result for the Desktop's one-filter/multi-value #h request, while one single-value #h filter per channel in the same request returns all six owner-visible workflow definitions. This confirms the compatibility failure and the fix shape in this PR.
• I rebased this patch locally onto current main and ran the affected package suites/checks.
• just desktop-tauri-test: 2,106 passed, 0 failed, 14 ignored.
• just desktop-test: 3,928 passed, 0 failed.
• just desktop-check, just desktop-build, just desktop-tauri-fmt-check, cargo fmt --all -- --check, just desktop-tauri-clippy, and cargo clippy -p buzz-relay --all-targets -- -D warnings: passed.
• cargo test -p buzz-relay: both new workflow regression tests passed; aggregate was 839 passed, 1 failed, 37 ignored. The sole failure was the unchanged api::mesh_demo::tests::demo_join_forwarded_arm_round_trips_echo test (504 vs expected 200), outside this PR's changed files.

GitHub currently reports the PR mergeable; required review is the remaining repository gate.