FEAT: Add CoPyRIT themes

[Roman Lutz (romanlutz)]

Description

Add seven built-in presets to the existing Theme menu: Raccoon, Jimothy, Pirate, Seattle Rain, Evergreen, Blueprint, and Night Sky. Seattle Rain uses a darker storm-gray palette. System, Light, and Dark remain available and keep their existing behavior.

A typed catalog supplies each preset's label, palette, background, menu entry, and saved ID. Adding another preset requires a catalog entry and an SVG, rather than changes throughout the app. One shared workspace background sits behind solid reading surfaces. Theme changes preserve drafts, and forced-colors mode hides decoration without forgetting the selected preset.

All seven background SVGs were drawn from scratch for this change with Copilot assistance. No artist illustrations, photographs, or stock wallpapers were copied or traced. Artwork provenance and factual Jimothy references are documented in the frontend README.

Tests and Documentation

• Added unit coverage for the catalog, measured text/button contrast, stored preferences, forced colors, menu selection, and workspace state preservation. The latest palette/provider run passed all 213 tests.
• Added browser coverage for palette rendering, reloads, forced colors, keyboard navigation, narrow touch screens, routes, and unsaved drafts. All nine palette cases were rerun successfully after the Seattle Rain update.
• Production build, changed-file ESLint, and commit hooks pass.
• Updated the GUI guide and added a contributor recipe for new presets and artwork provenance.
• No Python or notebook changes; JupyText was not needed.

Screenshots

Fresh captures of all seven presets at 1440 x 900, using empty preview data rather than a real campaign.

Theme	Screenshot
Raccoon
Jimothy
Pirate
Seattle Rain
Evergreen
Blueprint
Night Sky