Rework colour themes as greyscale + accent-only colour (ticket 032)

light/dark are now exact lightness mirrors of one true neutral scale,
grey is that same canonical neutral scale, and each named theme
(red/green/blue/yellow/pink/orange) derives its surface/border/text
greys by desaturating its own Tailwind hue at each scale step (HSL
S->0, lightness kept), leaving saturated colour only on --c-accent*.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

tickets/032-greyscale-theme-rework.md

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-# 032 — Rework colour themes as greyscale equivalents with accent-only colour
-
-## Summary
-
-Today's colour themes (`webapp/src/index.css`) only vary `--c-accent`/
-`--c-accent-soft`/`--c-accent-text` per theme (`html.red`, `.green`, `.blue`,
-`.yellow`, `.pink`, `.orange` at lines 97-130) — every theme shares the same
-dark `:root` surfaces/borders/text. `html.grey` (lines 75-95) is a Tailwind
-slate palette, which reads blue-tinted rather than neutral. The wishlist
-wants: `light`/`dark` as true black-and-white inverses of each other, `grey`
-as true neutral greyscale, and every other named theme (`red`/`green`/etc.)
-to desaturate its named hue to the matching greyscale lightness for
-surfaces/borders/text, reserving actual saturated colour for accents only.
-
-## Scope
-
-- `:root` (dark) and `html.light`: convert to a true black/white greyscale
-  scale (surfaces, borders, text all `#000`–`#fff` greys, inverted between
-  the two), replacing the current warm/cool-tinted neutrals.
-- `html.grey`: replace the slate-derived palette (lines 76-90) with the same
-  neutral greyscale scale as dark/light (i.e. `grey` becomes the canonical
-  greyscale theme other themes derive from), keeping only its own accent
-  distinct if one is still wanted.
-- `red`/`green`/`blue`/`yellow`/`pink`/`orange`: for each, desaturate that
-  hue to compute a matching greyscale lightness scale for
-  `--c-surface-*`/`--c-border-*`/`--c-text-*` (e.g. convert the hue to HSL,
-  zero the saturation, keep the lightness steps), then keep that theme's
-  actual saturated colour only on `--c-accent*`.
-- Document the derivation (e.g. a short comment or a small script/table) so a
-  new themed variant can be added consistently later.
-
-## Acceptance criteria
-
-- `light` and `dark` are literal greyscale inverses of each other (every
-  `--c-*` value in one is the other's lightness mirrored, no hue).
-- `grey` uses a neutral (zero-saturation) scale, not a blue-tinted one.
-- Each named colour theme's surfaces/borders/text render as neutral greys at
-  a lightness matching that colour's own luminance, with the named colour
-  visible only via `--c-accent*`.
-
-## Depends on
-
-None.
-
-## Depended on by
-
-- [033 — Downloadable/uploadable custom themes](033-custom-theme-download-upload.md)