feat(webui): Mono theme — pure white in light mode, pure black in dark mode

Adds a fourth accent alongside sakura/ocean/violet. Unlike those it is not just a
different hue: the coloured themes are glass surfaces (translucent cards with
backdrop-filter) floating over an animated gradient-mesh background, so setting
--card:#ffffff there still renders as a tinted grey. Mono therefore also switches
off the translucency and hides the blobs, so #ffffff is actually #ffffff and
#000000 is actually #000000, with greys carrying the hierarchy that hue carries
elsewhere. A side effect worth having: no backdrop-filter and no animated blobs
makes it the cheapest theme to render, which helps on weak GPUs and over remote
desktops.

Both light and dark variable blocks are defined, so the existing light/dark
toggle drives it with no extra wiring: light -> white, dark -> black.

Also fixes a latent theme bug found while checking contrast on black: the active
chart's grid baseline assigned the literal string "var(--line)" to
ctx.strokeStyle. Canvas 2D does not resolve CSS custom properties, so that was an
invalid colour the browser ignored, leaving the previous fillStyle (black) — an
invisible baseline on every dark theme. Colours used on a canvas now go through a
cssVar() helper.

Tests: TestUIThemeMatrix asserts every accent defines BOTH a light and a dark
block plus a picker button (a half-defined theme shows up as unreadable text, not
as an error); TestUIMonoThemeIsFlat pins the opaque surfaces and disabled blobs;
TestUICanvasColorsResolveVars fails if any ctx.strokeStyle/fillStyle is handed a
raw var().

Unrelated packaging fix in the same commit: dist:linux only built deb+AppImage
while build.linux.target listed rpm too, so `make gui-dist` silently skipped the
rpm that release builds are expected to produce. Makefile/README wording updated
to match.

README_EN.md

@@ -439,7 +439,7 @@ cd cmd/gui
 npm install
 make gui            # or npm run dev — dev run (needs make build first for bin/llmsproxy)
 make gui-deb        # local deb (share with other Linux users)
-make gui-dist       # deb + AppImage (linux)
+make gui-dist       # deb + rpm + AppImage (linux; rpm needs system rpmbuild)
 make gui-win        # Windows NSIS (host mingw + wine)
 make gui-win-docker # Windows NSIS, fully dockerized (no host mingw needed)
 ```