feat: inline markdown formatting (bold, italic, strikethrough, underline)

[oobagi]

Problem

The editor renders block-level markdown beautifully — headings, bullets, checklists, code blocks with syntax highlighting, quotes with bars. But inline formatting is completely absent: **bold**, *italic*, ~~strikethrough~~, and __underline__ all display as literal delimiter characters. For a markdown editor targeting developers, this is the most visible gap.

Context

The rendering pipeline processes blocks individually through three paths in internal/editor/render.go:

• renderInactiveBlock() (line 549) — blocks not being edited; renders static styled text via lipgloss
• renderViewBlock() (line 718) — view mode rendering; same static pattern
• renderActiveBlock() (line 88) — the block currently being edited; uses forked textarea on raw runes

All three follow the same pattern: get content → wrap text → apply block-level style → render. Inline formatting is a text transformation step inserted between wrap and render.

Existing pattern to extend: Checked checklist items already apply lipgloss.NewStyle().Faint(true) to entire wrapped text (render.go:616-619). This feature extends that from whole-block to span-level styling.

Wrap ordering: wrapText() calls textarea.Wrap() on raw runes. Wrap first → then parse and style inline formatting on wrapped text. Delimiters consume visual width during wrapping but are hidden after — acceptable, consistent with how glow and other terminal markdown renderers work.

Scope boundary: The active textarea (block being edited) shows raw markdown. Formatting appears when you click away. This matches Notion, Typora source mode, etc.

Approach: State-machine inline parser

Write a renderInlineMarkdown(text string) string function that walks text character-by-character, tracking delimiter open/close state, and emits lipgloss-styled spans.

• Handles nesting correctly (***bold italic***, **bold with *italic* inside**)
• No external dependencies — pure string processing + lipgloss
• Ignores _ in snake_case (only match __ at word boundaries or standalone)
• Extracted to internal/format/ for reuse and easy unit testing
• Skips code blocks entirely (already syntax-highlighted)

Tasks

• Add RenderInlineMarkdown(text string) string in internal/format/inline.go
• Support **bold**, *italic*, ~~strikethrough~~, __underline__, and nesting
• Handle edge cases: empty delimiters (****), snake_case not treated as underline, unmatched delimiters rendered literally
• Insert call in renderInactiveBlock() after wrapText(), before block-type styling — skip for code blocks
• Insert call in renderViewBlock() at the same pipeline position — skip for code blocks
• Active blocks unchanged (show raw markdown)
• Add unit tests for the inline parser

Test plan

• Inactive paragraph with **bold** renders bold, no asterisks visible
• View mode renders all four formatting types correctly
• ***bold italic*** renders with both styles
• **bold with *nested italic* inside** renders correctly
• Active block shows raw delimiters (no formatting)
• Code blocks show literal **text** without formatting
• snake_case_variables are NOT treated as underline
• Unmatched **text renders literally (no crash, no style leak)
• All existing tests pass

Scope

Type: enhancement
Size: medium