feat: block text alignment (left, center, right)

[oobagi]

Problem

There's no way to change the alignment of a block. All content renders left-aligned. Users should be able to set blocks to left, center, or right alignment.

Context

The block model (internal/block/block.go) currently has three fields: Type, Content, and Checked. Rendering in internal/editor/render.go always left-aligns text within the available width. The command palette (internal/editor/palette.go) handles block type changes but has no concept of block properties beyond type.

Key files:

• internal/block/block.go — Block struct, BlockType enum
• internal/block/parse.go — Markdown → Blocks parsing
• internal/block/serialize.go — Blocks → Markdown serialization
• internal/editor/editor.go — Keyboard handling, block operations
• internal/editor/render.go — Active/inactive/view rendering
• internal/editor/palette.go — Command palette
• internal/editor/undo.go — State snapshots (includes block data)

Approach

Data model: Add an Align field to Block (enum: Left, Center, Right, default Left).

Markdown persistence: Serialize as an HTML comment above the block — <!-- align:center -->. This is invisible to other markdown renderers and won't corrupt content. On parse, detect and strip the comment, setting the block's alignment.

Rendering: Pad lines with leading spaces to shift content center or right within the available width. Applies to both active block, inactive block, and view mode rendering paths in render.go.

UX:

• Add /left, /center, /right entries to the command palette
• Consider a keyboard shortcut (single key that cycles, or direct shortcuts — avoid Ctrl+E/Ctrl+A per macOS terminal conflicts)
• Show alignment in the gutter or as a subtle indicator

Tasks

• Add Align field and Alignment enum to internal/block/block.go
• Update internal/block/parse.go to detect <!-- align:X --> comments and set alignment
• Update internal/block/serialize.go to emit alignment comments for non-left blocks
• Update internal/editor/render.go — pad lines for center/right in all three render paths
• Add alignment entries to the command palette in internal/editor/palette.go
• Add keyboard shortcut for alignment cycling in internal/editor/editor.go
• Update editorState in internal/editor/undo.go if needed (alignment is stored on blocks, so it should already be captured)
• Add tests for parse/serialize round-tripping with alignment comments

Acceptance criteria

• Blocks can be set to left, center, or right alignment via palette or shortcut
• Alignment persists across save/load (markdown round-trip)
• Alignment renders correctly in edit mode and view mode
• Default is left-aligned; no <!-- align:left --> comment emitted for default
• Undo/redo captures alignment changes