feat: large ASCII-art headings in view mode

[oobagi]

Problem

Headings in view mode are just bold text — they don't feel visually distinct or impactful. mdfried-style large titles would make view mode feel more like a polished reading experience.

Context

View mode renders H1/H2/H3 as plain styled text via lipgloss in renderViewBlock() (internal/editor/render.go:713-723). Heading styles are defined in internal/theme/theme.go as HeadingStyle{Text: TextStyle{Bold, Italic, Color, ...}}. View mode centers content within viewMaxWidth = 72 (internal/editor/editor.go:258).

mdfried uses Kitty text sizing protocol or cosmic-text rasterization, but both require terminal graphics protocol support. For a Go/Bubble Tea app, figlet-style ASCII art is the practical approach — works in all terminals, no protocol detection needed, and fits the aesthetic.

Go libraries: github.com/common-nighthawk/go-figure or github.com/lukesampson/figlet for FIGlet font rendering.

Key files:

• internal/editor/render.go — renderViewBlock() (lines 692-808), heading render cases
• internal/theme/theme.go — HeadingStyle struct, theme presets
• internal/editor/editor.go — viewMaxWidth constant, view mode logic

Approach

Use a Go FIGlet library to render H1 text as multi-line ASCII art in view mode only. H2/H3 stay as styled text (large ASCII art for all heading levels would be noisy).

• Add a FIGlet font field to HeadingStyle (e.g., ArtFont string) so themes can pick their own font (or disable with empty string)
• In renderViewBlock(), detect H1 + non-empty ArtFont → render via figlet instead of plain text
• Center the ASCII art output within viewMaxWidth
• Edit mode stays unchanged — ASCII art headings are a view-mode-only enhancement
• Truncate or fall back to plain text if the ASCII art exceeds terminal width

Tasks

• Add a Go FIGlet dependency (evaluate go-figure vs figlet vs vendoring a minimal implementation)
• Add ArtFont string field to HeadingStyle in internal/theme/theme.go
• Set default FIGlet fonts for H1 across theme presets (pick fonts that match each theme's personality)
• Update renderViewBlock() in internal/editor/render.go to render H1 as ASCII art when ArtFont is set
• Handle edge cases: long titles that overflow width, empty content, narrow terminals
• Ensure ASCII art respects theme accent color

Acceptance criteria

• H1 blocks render as multi-line ASCII art in view mode
• Each theme can specify its own FIGlet font (or disable with empty string)
• Edit mode rendering is unchanged
• ASCII art is centered within the view mode content area
• Graceful fallback for titles too wide for the terminal

[oobagi]

Closing as not planned — FIGlet/ASCII art fonts don't produce good enough results for the aesthetic we're going for.