initial colors and theme improvements (#7009)

## What this does

Consolidates the frontend's colour/theme system into a small,
well-defined token layer and reworks the theme picker. The goal was a
minimal, scalable set of semantic tokens that the editor **and** the
Processor/portal (and Storybook) all share, plus a theme model that's
easy to reason about.

## Token architecture (`core/theme/`)

A four-file layer, imported once via `index.css`:

| File | Role |
|---|---|
| `primitives.css` | The raw palette — the **only** place literal
colours live (neutral ramps `--p-gray-*`/`--p-zinc-*` + status hues). |
| `colors.css` | ~21 semantic `--c-*` tokens (surfaces, text, borders,
primary, status) mapped from primitives per theme. **Reference these.**
|
| `compat.css` | Legacy names (`--bg-*`, `--text-*`, `--color-*`)
aliased onto `--c-*` via `:root:root` so ~200 existing files keep
working. |
| `dimensions.css` | All non-colour tokens (spacing, radius, z-index,
type, motion) — single source, resolving prior collisions. |

A blocking linter (`scripts/lint/theme-lint.mjs`, run in
`frontend:lint`) enforces "literals only in `primitives.css`" within
`core/theme/`, and has a non-blocking WCAG contrast report. See
`core/theme/README.md`.

## Theme model

- **Mode** (`light` / `dark` / `system`) and **accent** are independent.
Each mode has its own accent (`lightPrimary` / `darkPrimary`).
- The editor is always `data-app-theme="custom"`; `ThemeProvider`
injects the accent as `--user-primary` and sets `data-accent`.
- **Two accent states:**
- A **colour** (preset or custom hex) → tints every surface that hue
(whole-app theming).
- The **`default`** sentinel → neutral surfaces (white/grey light, zinc
black/grey dark) with blue buttons, no tint. (`data-accent="default"`
opts surfaces out of the tint.)
- Accent contrast guardrails (`utils/customPrimary.ts`): lightness
clamps so an accent can't collapse into the base, a contrast-picked
on-primary foreground, and an accent-as-foreground variant so accent
text is never dark-on-dark.

## Theme picker (Settings → General)

- 3×5 grid: a distinct **Default** icon chip (not a colour) + 14 curated
accents, in a dropdown per mode.
- **Custom** colour via the shared `ColorInput`, with a live gamut clamp
(`clampValue`) that refuses white/grey/black — the picker handle sticks
at the boundary and preserves the working hue at achromatic extremes.
- "Restore theme to default" resets both modes.

## Other

- Dark mode is a true neutral zinc (no navy "midnight" tint); the
Mantine dark ramp and Tailwind dark channels were neutralised to match.
- Pre-paint inline script in `index.html` applies theme + accent before
first paint (no FOUC); portal and editor now share the same
`preferences.theme` source of truth.
- High-visibility surfaces migrated to tokens (FAB, landing upload
buttons, portal hero banners); scattered per-component colour swaps were
intentionally **left for a follow-up** to keep this PR focused.

## Testing

- `task frontend:check:all` (typecheck all variants + eslint + prettier
+ colour-lint) green.
- Verified light/dark, default vs tinted accents, and the custom clamp
via computed styles in the dev preview.

> Note: the `prerender-og` build step failing in the e2e/deploy jobs is
unrelated to this diff — it's in `vite.config.ts` (untouched here) and
builds cleanly locally.

.taskfiles/frontend.yml

@@ -203,6 +203,19 @@ tasks:
     cmds:
       - task: lint:eslint
       - task: lint:dpdm
+      - task: lint:colors
+
+  lint:colors:
+    desc: "Enforce theme tokens — colours in core/theme route through the palette"
+    deps: [install]
+    cmds:
+      - node editor/scripts/lint/theme-lint.mjs
+
+  contrast:
+    desc: "Report low-contrast theme token pairs (warning only, never blocks)"
+    deps: [install]
+    cmds:
+      - node editor/scripts/lint/theme-lint.mjs contrast
 
   lint:eslint:
     desc: "Run ESLint linting"