feat(themes): add Boreal + make theme appearance per-theme

Boreal — a Nord × Everforest × Dracula dark theme: a boreal forest at
night under the aurora. Nord's frost bones, Everforest's comfort, Dracula's
jewel-tone violet/cyan as the aurora light.

Palette (WCAG-checked, all 24 themes × 7 pairings pass): frost-slate base,
soft-vivid violet accent (#B79BE8, 6.35 on base) + frost-cyan accentAlt
(#86C7C0, 7.84 AAA), sage-green/gold/coral semantics, text 10.66 AAA.

Bespoke, non-standard surfaces:
- Waybar whole-swap (jsonc + css): a floating "aurora frost" bar of three
  frosted-glass pills; launcher+clock left, workspaces centre, and the whole
  right side is ONE continuous pill — the system tray shares its glass with
  the status modules, no separators (the seamless-tray requirement). Full
  module set (parity). GeistMono.
- Rofi: a frosted list base (used by every menu), plus a 4×3 large-icon grid
  for the app launcher only.
- Typography GeistMono Nerd Font (57 MB, new); Iosevka was rejected at 1.1 GB.

Per-theme appearance (new): a preset now carries a full fonts+ui block, like
border, so a theme can ship a bespoke font / terminal opacity / border
thickness / rounding and a switch always replaces it — no leak into the next
theme (verified: boreal→everforest resets mono/rounding/opacity). All 23 other
presets get the current defaults (idempotent — zero visual change). Boreal
runs GeistMono + 0.90 terminal opacity + 3px borders + 12px rounding.

Grid launcher (new, backward-compatible): drun renders via `-theme launcher`;
HM writes ~/.local/share/rofi/themes/launcher.rasi which defaults to
`@import "custom.rasi"` (every other theme's launcher = its list, unchanged),
and a theme may override it (Boreal's grid). Text menus keep the base list.

Wallpaper: an original procedurally-generated aurora-over-treeline (ImageMagick),
palette-matched.

V1: nix flake check --no-build green; strict JSON on every preset; contrast
checker green; the real rofi binary parses the base rasi and resolves
`-theme launcher` + @import + grid overlay; end-to-end eval with boreal active
resolves the waybar whole-swap (tray in the right cluster), the rofi grid
launcher, and the bespoke fonts/opacity/geometry reaching nomarchy.theme, with
no cross-theme contamination. NOT done: on-screen render of the desktop
(V2/V3) and preview.png (deferred to on-hardware capture).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

themes/flexoki-light.json

@@ -22,6 +22,23 @@
     "active": "accent",
     "inactive": "overlay"
   },
+  "fonts": {
+    "mono": "JetBrainsMono Nerd Font",
+    "ui": "Inter",
+    "size": 11
+  },
+  "ui": {
+    "gapsIn": 5,
+    "gapsOut": 12,
+    "borderSize": 2,
+    "rounding": 10,
+    "iconSize": 36,
+    "activeOpacity": 1.0,
+    "inactiveOpacity": 0.95,
+    "terminalOpacity": 0.96,
+    "blur": true,
+    "shadow": true
+  },
   "ansi": [
     "#100F0F",
     "#D14D41",