feat: embed block type (reference another note inline)

[oobagi]

Problem

There's no way to reference another note. Users who organize related information across multiple notes have no way to cross-link them.

Goal

Add a note link block type that references another note and opens it in a modal overlay when clicked.

Markdown format

![[notebook/note-title]]

Behavior

• Edit mode: Renders as editable raw syntax. Insertable via / command palette with notebook/note picker.
• View mode: Renders as a styled link/pill showing the note title. Clicking opens the referenced note in an iOS-style modal overlay (slides up, dimmed background, dismissable).
• Missing references: Show a "Note not found" badge instead of the link pill.

Tasks

• Add Embed to BlockType enum
• Parse ![[path]] syntax in parse.go
• Serialize back to embed syntax
• Round-trip tests
• Add to / command palette with notebook/note picker
• Resolve references at render time — check if referenced note exists
• View mode: render as styled link pill
• View mode: open referenced note in modal overlay on click
• Modal overlay: iOS-style slide-up sheet with dimmed background, read-only note content, dismiss via Esc or click outside
• Handle missing references gracefully (styled "Note not found" pill)
• Add EmbedStyle to theme system (pill color, modal style)

Design considerations

• This is a link, not inline content — no need for circular detection or depth limits
• Modal shows the full note read-only; user can dismiss and return to the current note
• Keep it simple: no navigation stack within the modal (v1)

Scope

Type: enhancement
Size: medium