Replace internalContent prop sync effects with $derived

[malibio]

Overview

Replace $effect blocks that sync internalContent from content prop with $derived pattern.

Problem Statement

Four node components use the anti-pattern of syncing a prop to internal state via $effect:

let internalContent = $state(content);
$effect(() => {
  internalContent = content;
});

This is unnecessary state duplication. The internalContent can simply be derived from content.

Files to Modify

1. calendar.svelte (line 39) - sync external value to internal
2. code-block-node.svelte (line 76) - sync internalContent
3. ordered-list-node.svelte (line 54) - sync internalContent
4. quote-block-node.svelte (line 55) - sync internalContent

Proposed Solution

Replace with $derived:

// BEFORE
let internalContent = $state(content);
$effect(() => {
  internalContent = content;
});

// AFTER
const internalContent = $derived(content);

Or if mutation is needed, use bind:content in parent and remove internal state entirely.

Acceptance Criteria

• calendar.svelte $effect replaced with $derived
• code-block-node.svelte $effect replaced with $derived
• ordered-list-node.svelte $effect replaced with $derived
• quote-block-node.svelte $effect replaced with $derived
• 4 $effect blocks eliminated
• All node types render correctly
• Edit mode works correctly
• Tests pass (bun run test:all)
• Quality checks pass (bun run quality:fix)

Related Issues

• Epic: Eliminate $effect Anti-patterns - Embrace Svelte 5 Reactivity #623 (parent epic)

[malibio]

Test baseline: Frontend 1469 passed (74 files), Rust 531 passed (16 ignored)

[malibio]

Implementation Notes (from PR #696 review)

The actual implementation differs from the original proposal, for valid technical reasons:

Original Proposal:

const internalContent = $derived(content);

Actual Implementation:

let internalContent = $state(content);
// $effect removed entirely, not replaced with $derived

Why this approach is correct:

1. Mutable state required: These components modify internalContent within handleContentChange handlers (injecting language codes, adding prefixes). Using $derived(content) would make it read-only, breaking the mutation logic.

2. The $effect was truly redundant: The sync $effect(() => { internalContent = content; }) served no purpose because:

   • Initial sync: Already handled by $state(content) initialization
   • User edits: Flow through event handlers, not prop changes
   • External changes: Handled by component remount

3. calendar.svelte excluded: Its $effect is a necessary workaround for bits-ui's Svelte 5 binding limitations (two-way binding bridge pattern) - a fundamentally different use case.

Final count: 3 $effect blocks removed (not 4) since calendar.svelte was correctly preserved.