feat(app): add Catppuccin Mocha, Nord, and Dracula theme presets

[griffinwork40]

What this adds

Three community themes ported verbatim from their official terminal specifications:

Preset	Config string	Background	Foreground	Source
Catppuccin Mocha	"catppuccin-mocha"	#1e1e2e	#cdd6f4	catppuccin/catppuccin
Nord	"nord"	#2e3440	#d8dee9	nordtheme.com
Dracula	"dracula"	#282a36	#f8f8f2	draculatheme.com

All three are MIT-licensed community palettes. Each is a verbatim port — the upstream hex values are transcribed exactly, including their known weaknesses, because a preset's job is to be the thing it claims to be.

Usage

{ "theme": { "preset": "catppuccin-mocha" } }
{ "theme": { "preset": "nord" } }
{ "theme": { "preset": "dracula" } }

Contrast gate results

bash Scripts/check-theme-contrast.sh — ALL-OK: 474 assertions passed (up from 345 before this PR).

The 129 new assertions cover:

• Hex parseability for all three palettes (background, foreground, cursor, selection, 16 ANSI slots each)
• Config-string round-trip: named("catppuccin-mocha"), named("nord"), named("dracula") and their hyphen-stripped / underscored / uppercased variants
• Tab-strip chrome legibility for all three palettes (rail contrast, active/inactive label Lc, active/inactive separation)
• Syntax role legibility for all three palettes (five roles × three palettes)
• Selection pairing readability

Documented gaps (pinned, not silently "fixed")

Catppuccin Mocha and Nord: all eight chromatic normal/bright ANSI pairs are identical (dE-OK 0.000). This is a known property of the upstream palettes — the same trait Tokyo Night (already shipped) exhibits. Documented in ThemeValues.swift's umber doc comment as the measured failure mode. The gate does not hold community ports to Umber's ring-separation floor (only Umber is designed to that spec).

Nord keyword color (PINNED): nord9 (#81a1c1) measures APCA Lc 43.6 on nord0 (#2e3440) — 1.4 points below the Lc 45 floor for any-size readable text. This affects the keyword syntax role in the file editor. A pinned assertion in check-theme-contrast-syntax.swift holds it to ≥ 40, so silent regression to truly unreadable territory is still caught. The correct fix would be Nord upstream choosing a slightly brighter blue; substituting a colour the upstream palette doesn't ship would make this not-Nord.

Nord and Catppuccin Mocha comment colors: ANSI 8 is below Lc 45. The existing commentColour() fallback in SyntaxPalette.swift applies and composites toward the foreground at 0.72 alpha, lifting the comment to a readable Lc. This is the same mechanism that already handles Tokyo Night and afk-dark.

Files changed

• Sources/Umber/ThemeValues.swift — three new ThemePalette statics, added to .all (333 LOC, ceiling 350)
• Sources/Umber/Theme.swift — three Theme bridge statics
• Sources/Umber/StarterConfig.swift — updated preset list in the commented config template
• Scripts/check-theme-contrast-registry.swift — expectedNames expanded to include the three new presets
• Scripts/check-theme-contrast-syntax.swift — Nord keyword gap pinned (same pattern as classic-repaired's comment gap)

Verification

bash Scripts/check-theme-contrast.sh   # ALL-OK  474 assertions passed
bash Scripts/check-file-size.sh        # all 116 files under 350 LOC
swift build                             # Build complete