feat(ui): add classic/modern/phosphor/brutalist theme skins

[gnacho]

Summary

New theme system for the EasyZFS UI: four distinct visual skins switchable from Settings → Appearance, keeping the classic look intact (and as default for existing users). Theme and mode are now separate controls, and each family ships its own accent palette.

Closes #130.

Themes

• Classic — the existing light/dark/system look (default for current users).
• Modern — default for new installs (dark/light per OS): violet accent, borderless cards with soft elevation, pill active nav, subtle radial halo.
• Phosphor — CRT/terminal: JetBrains Mono, zero radius, scanlines + grid, HUD-cornered cards, segmented meters, squared uppercase buttons/badges, phosphor-green identity.
• Brutalist — cream paper, 2px black borders, hard offset shadows, signal-yellow primary, marker-underlined titles, black active nav with yellow text.

Behavior

• Family × Mode: pick a family (Classic/Modern/Phosphor/Brutalist) and, for Classic & Modern, a mode (System/Light/Dark). Phosphor and Brutalist are fixed.
• Accents per family: each theme offers its own palette with its default accent preselected; the accent picker works on every theme while state colors stay semantic.
• Persistence via easyzfs-theme (family) + easyzfs-theme-mode (mode); migration from the previous combined key. easyzfs-accent-<family> for per-family accent overrides.
• Also: slightly larger type in the Dashboard / disk-health sections, and the rebuild banner uses a blue (info) accent in the Modern theme.

Verification

• npm run build OK, go test ./internal/... OK.
• Playwright (local MOCK/DEMO): default modern-light; Classic → dark/''; Phosphor → phosphor; accent per family applies (Modern violet, Phosphor green); dropdown + mode Seg; no page errors.

Docs

• Changelog EN + ES added for v2.9.22.