Add content types filter to mobile channel editor

[disnet]

Summary

• Mobile channel editor (MobileFilterSheet.svelte Channel tab) now exposes the same Content Types checklist that the desktop FilteredViewModal already offers, so feed channels can be narrowed by source type from a phone.
• Visibility and save-payload gating mirror desktop exactly: shown for feed channels except when sources are manual + include-only; cleared on save in that combo.
• Existing channels load their typeFilter selections back into the mobile editor.

Test plan

• cd frontend && npm run check (passing locally — 0 errors)
• Manual viewport-narrow smoke:
  • New feed channel, Sources = All → Content Types section visible; toggle a checkbox, save, reopen → persists
  • Switch to Manual + Include only → Content Types section hides
  • Switch back to All → previously checked types still in state (only the save payload is gated, not the UI state)
  • Switch to Saved channel type → Content Types hidden
  • Switch to Smart mode → Content Types visible
  • Edit a desktop-created channel that already has typeFilter → checkboxes reflect existing selections
  • Saved feed view honors the filter (article list narrows to selected types)

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