Remove unsafe type assertions from component merging

[Copilot]

The mergeDeeply function in src/components/merge.ts used type assertions to coerce union types, triggering 6 no-unsafe-type-assertion warnings.

Changes:

• Replaced generic mergeDeeply with dedicated type-safe merge functions per component type
• mergeBlockComponent, mergeListComponent, mergeListItemComponent handle union types (function | object) with proper type guards
• mergeMarksComponent, mergeTypesComponent handle record types without assertions

Before:

return {
  block: mergeDeeply(parent, overrides, 'block') as PortableTextHtmlComponents['block'],
  // ... 5 more unsafe assertions
}

After:

return {
  block: mergeBlockComponent(parent.block, overrides.block),
  list: mergeListComponent(parent.list, overrides.list),
  // ... type-safe merges
}

Each merge function's return type is explicitly typed, allowing TypeScript to verify type safety without assertions.

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Latest commit: 55ce3bd

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