fix(desktop): keep channel-sections scoped to the active community

[changsheng1224]

Problem

Desktop persists and publishes the channel-sections layout of the
previously active community into the newly active community after a
workspace switch. Community A's kind:30078 d=channel-sections event (and
the matching localStorage key) can end up with channel UUIDs and section
objects that exist only on community B.

This is a persistence/publish bug, not only a render bug: the wrong assignment
map is written to the other community's relay, so it survives restarts and
follows the user to a new machine.

Cause

Section state is keyed …:<pubkey>:<relayUrl>, but in-memory writers and
first-sync seed-publish were not reliably fenced to the active community
before channels were ready. A stale store (or a polluted local blob) could be
persisted/published under the new relay URL, including while the channel
allowlist was still empty and filtering was a no-op.

Fix

• Fence persist/publish by community scope generation so prior-community
  writers cannot land under the new relayUrl.
• Scope assignments to known channel ids of the active community before
  localStorage write and kind:30078 publish; drop section folders that only
  held foreign-channel assignments.
• Defer bootstrap seed-publish until the channel list is ready
  (channelsReady), then heal already-polluted local state and republish the
  cleaned blob.
• Filter the rendered sections/assignments the same way so a stale payload
  cannot resurrect the symptom.

Related

• Fixes Desktop: channel sections of one community are published into another community's relay after a workspace switch #7207.
• Desktop counterpart of fix(mobile): channel section folders bleed across linked workspaces #5762 (mobile section folders across linked
  workspaces).
• Overlaps file-wise with open fix(sidebar): converge channel sidebar state across devices #6525 (fix(sidebar): converge channel sidebar state across devices), which rewrites the same hook for multi-device
  LWW/outbox convergence but does not close the cross-community channel-id
  leak. This PR is intentionally narrower and landable on current main; if
  fix(sidebar): converge channel sidebar state across devices #6525 merges first, this will need a rebase.

Testing

• pnpm exec node --import ./test-loader.mjs --experimental-strip-types --test
  on:
  • desktop/src/features/sidebar/lib/channelSectionsHelpers.test.mjs
  • desktop/src/features/sidebar/lib/useChannelSections.test.mjs
  • desktop/src/features/sidebar/lib/channelSectionsSync.test.mjs
  • desktop/src/features/sidebar/lib/channelSectionsStorage.test.mjs
    (58 passed)
• Falsifiability: the new Desktop: channel sections of one community are published into another community's relay after a workspace switch #7207 hook tests fail against origin/main
useChannelSections.ts and pass with this change
• pnpm exec playwright test --project=smoke tests/e2e/community-rail.spec.ts -g 7207
  (1 passed — organize on B, switch to A, assert A's store and post-switch
  sign_event payloads carry no B section layout)

[github-actions]

🔐 Codex Security Review

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