feat: allow block type transformation via palette on non-empty blocks

[oobagi]

Problem

There's no way to change a block's type once it has content. The / palette only opens on empty blocks (ta.Value() == ""), so users who want to turn a paragraph into a heading — or a bullet into a quote — must clear the block, change its type, then re-type the content.

This is a common editing operation (realize a paragraph should be a heading, promote/demote headings, convert to a list after drafting). The infrastructure already supports it — applyPaletteSelection preserves content for all non-Divider types — but the opening gate blocks it.

Context

• Palette open condition (internal/editor/editor.go:1184-1188): requires ta.Value() == "", ta.Line() == 0, ta.LineInfo().ColumnOffset == 0
• applyPaletteSelection (internal/editor/editor.go:729-743): already changes type and preserves content; only clears content for Divider
• Backspace at pos 0 already converts lists → paragraph, showing the pattern exists for non-empty type changes
• Palette filtering, navigation, and selection all work independently of content state

Possible approaches

Approach A: Extend / trigger to position-0 on non-empty blocks

• How: Remove the ta.Value() == "" gate. Keep the position-0 requirement so / mid-sentence still types a slash.
• Pros: Single mental model ("go to start of block, type /, pick a type"). Mirrors Notion. Minimal code change.
• Cons: Need to ensure / isn't inserted into the block text when triggering the palette.
• Files: internal/editor/editor.go (open condition), internal/editor/palette.go (maybe hide Divider when content exists)

Approach B: Separate shortcut (e.g., Ctrl+/) for non-empty blocks

• How: Add a dedicated keybind that opens the palette regardless of content or cursor position.
• Pros: No ambiguity about when / is a character vs. a command.
• Cons: Two ways to do the same thing. More to document. Conflicts with macOS terminal remapping concerns.
• Files: internal/editor/editor.go

Recommended: Approach A — simpler, consistent, less to learn.

Tasks

• Remove ta.Value() == "" from palette open condition (keep pos-0 check)
• Ensure the / character is not inserted into block content when palette opens
• Hide Divider from palette options when the current block has content (avoids silent data loss)
• Visually indicate the current block type in the palette (dim or checkmark)
• Verify undo reverts a type transformation as a single operation
• Add tests: palette opens on non-empty block at pos 0, content preserved after type change, / still types normally mid-text

Test plan

• Place cursor at position 0 of a block with text, press / — palette opens
• Select a different type — block type changes, all content preserved
• Place cursor mid-text, press / — literal slash inserted, no palette
• Open palette on non-empty block — Divider option is hidden
• Select the block's current type — no-op, palette closes
• Undo after type change — type and content revert
• Multi-line block converted to heading — content preserved

Scope

Type: enhancement
Size: small–medium