feat: cascade list type changes across related list items

[oobagi]

Problem

When changing the type of a list item via the palette (e.g., BulletList → NumberedList or Checklist), only the active block changes. This breaks list consistency — you end up with a mixed list where one item is a different type than its siblings/parents/children, which is almost never what the user wants.

Context

• Block types: internal/block/block.go:19 (BulletList, NumberedList, Checklist), with IsListItem() at line 73.
• Palette type conversion is driven through internal/editor/palette.go (openForBlock, defaultPaletteItems) and applied in internal/editor/editor.go where palette selection mutates m.blocks[m.active].Type.
• Indent is tracked per-block via Block.Indent (see nested lists work from 2026-04-06).

Approach

When the palette selects a new list type and the active block IsListItem():

1. Walk outward from m.active to find the full contiguous list group — siblings at any indent level, plus children (deeper indent) and parents (shallower indent) that are still list items.
2. Stop the walk at the first non-list-item block in either direction.
3. Apply the new list type to every block in that group, preserving each block's Indent and Content (and Checked where applicable — dropping check state when converting to non-Checklist).

Wrap in a single pushUndo() so the whole cascade is one undo step.

Tasks

• Add a helper (e.g., listGroupRange(idx int) (start, end int)) that returns the span of the contiguous list group containing idx.
• On palette selection of a list type while m.blocks[m.active].IsListItem(), apply the new type across [start, end].
• Handle the reverse: converting from list → paragraph via palette should also cascade (or explicitly not — decide and document).
• Tests in internal/editor/palette_test.go covering flat lists, nested lists, and mixed-indent groups.

Acceptance criteria

• Changing list type on any item in a list converts the entire contiguous list group.
• Nested children (deeper Indent) and parents (shallower Indent) in the same group convert together.
• Non-list blocks adjacent to the list are untouched.
• A single Ctrl+Z undoes the full cascade.