fix(desktop): show workflows across joined channels

[aaron-acev]

What changed

• build one kind 30620 workflow-definition filter per joined channel
• keep those filters in one relay query, relying on Nostr's OR semantics across filters
• add a regression test that requires each filter to carry exactly one #h value

Why

The Workflows overview batched all joined channel IDs into one #h array.
Deployed relays may treat that array as one value, so workflows outside the
first channel disappear even though the CLI can read them. Sending one filter
per channel preserves the single relay round trip while making cross-channel
matching unambiguous.

User impact

The Desktop Workflows overview can show definitions from every joined channel
instead of only the first matching channel.

Validation

At exact head 6c3b65c47f9c294115cee2702278ed324533cba2:

• just desktop-ci — PASS (3,906 frontend tests; 2,089 Tauri tests + 3 diagnostics; zero failures; frontend checks/build, Tauri fmt/check)
• just desktop-tauri-clippy — PASS with warnings denied
• semantic patch ID remained a267ba514e489143e38a344f923f01a93f73ca55 after rebasing onto current origin/main 19d57b0d46baa55814ac737041a36d0b405c9f64

Originating Buzz thread:
buzz://message?channel=582358dd-f6db-455c-a451-3c29d006df08&id=2fb67c6cb57f7b1e34701b48a81806a604b57ae8562b026d7151c6a655d11242

[aaron-acev]

Ready for review at exact head 6c3b65c47f9c294115cee2702278ed324533cba2.

Independent implementation review found no code blocker. Local exact-head validation passed just desktop-ci (3,906 frontend tests; 2,089 Tauri tests plus 3 diagnostics), all-target Tauri clippy with warnings denied, the focused 12-test Rust workflow suite, formatting, and diff hygiene.

The upstream CI and Desktop Release Candidate runs currently show action_required with zero jobs because this is a fork PR. A maintainer still needs to approve those workflows and provide the required review; no merge is requested before both gates pass.