fix(desktop): paginate complete channel directory

[tlongwell-block]

Summary

• page the channel-membership and open-channel metadata queries instead of relying on one oversized request
• use the relay's composite (until, before_id) cursor so same-second events cannot be skipped
• preserve the existing paginated people search/member-picker path

Validation

• cargo test --manifest-path desktop/src-tauri/Cargo.toml (1119 passed, 11 ignored; integration diagnostic 3 passed)
• pre-commit hooks passed
• pre-push desktop, desktop-Tauri, and mobile suites passed; unrelated root buzz-db suite is blocked because local rustc 1.89 is older than sqlx 0.9's rustc 1.94 requirement

[tlongwell-block]

Local high-volume validation

Followed TESTING.md and crates/buzz-cli/TESTING.md against an isolated local stack (Postgres :55432, Redis :56379, relay :3130; no Desktop data shared).

Validation found and fixed an additional relay-side prerequisite: get_accessible_channel_ids capped its open-channel access scope at 1,000, filtering older channel metadata before the desktop paginator could see it. Commit 0a94296bc8eede0b4610ccfa0c65c5e45453780e removes that internal cap and adds a 1,001-channel DB regression test.

Results after the fix:

• Seeded 1,602 open channels; 1,601 metadata events shared the exact same timestamp to exercise (until, before_id) tie-breaking.
• A single raw /query with limit: 5000 still returned exactly 1,000, confirming the relay event-query clamp remains intact.
• The actual patched desktop get_channels implementation returned 1,602 / 1,602 channels, including seed-0001 and seed-1601, in 250.5 ms locally.
• DB regression: accessible_channel_ids_are_not_truncated_at_one_thousand passed against the isolated Postgres instance.
• Normal usage smoke passed end-to-end: create channel, get/list metadata, add member, join as the second identity, send root, send reply, read messages, read thread, list members. The sequence completed in 0.252 s and returned 2 members.
• Full buzz-db lib suite: 79 passed, 64 ignored.
• Full desktop Tauri suite: 1,119 passed, 11 ignored; diagnostics 3/3 passed.
• cargo clippy -p buzz-db --lib -- -D warnings, pre-commit, and pre-push passed. Pre-push also reran desktop/Tauri/mobile suites.

Verified local and remote PR head: 0a94296bc8eede0b4610ccfa0c65c5e45453780e.