feat(ui): warm light theme and pre-mixed status fills

- Light mode moves to Claude's ivory/terracotta family: page #e8e5da, cards
  #f5f3ec (a notch below Claude's ivory, no pure-white step), warm
  borders and card shadow; dark mode is untouched.
- The surface ramp rises in lightness with its index in BOTH themes, so a
  card sits brighter than the page and every hover lifts instead of dims.
- Light --accent #a34a28 (the brand #cc785c is 3.1:1 on ivory); --fg-muted
  held at >=4.5:1; new --syn-* syntax tokens, dark keeps its pastels.
- Status/accent fills (bg-*/N, border-*/N) compile to color-mix against
  --tint-base instead of rgb(/a), so a chip is one colour on every surface.
- Hardcoded dark-only hovers (hover:text-white, hover:bg-white/5) replaced
  by tokens in DateTimePicker and SessionDrillIn.

client/README.md

@@ -53,6 +53,16 @@ The client is a single-page application (SPA) built with modern web technologies
 - **TypeScript 5.7** - Full type safety across components, utilities, and API contracts
 - **Vite 6.1** - Lightning-fast HMR during development, optimized production builds
 - **Tailwind CSS 3.4** - Utility-first CSS framework; colors are CSS-variable-backed tokens (`darkMode: "class"`) so a Dark/Light toggle (next to the language switcher in the sidebar, `useTheme` hook) re-themes the whole app by flipping one class
+
+  **The surface ramp means elevation, and it runs the same direction in both themes: a higher index is LIGHTER in dark and LIGHTER in light.** So in light mode the page is the *darkest* step (`surface-0`, `#e8e5da`) and a `.card` is brighter than the page it sits on (`surface-3`, `#f5f3ec`), which is also what makes every `hover:bg-surface-N` lift rather than dim. Light mode originally used Radix slate in its native direction — index up meant darker — which inverted the whole language: a near-white page (the glare complaint) under grey cards that read as recessed. Never "fix" a light-mode colour by swapping which step a component uses; fix the value of the step.
+
+  Other rules that fall out of this:
+  - **Light mode's hue family is warm, dark mode's is cool.** The light ramp is Claude's own ivory/terracotta (`#e8e5da` → `#fcfbf7`, warm neutrals, warm `--shadow-card`), not Radix slate: a cool near-white page reads as glare at any lightness, because the brightest thing on screen is also the bluest. Dark mode stays slate + blue — a warm accent on a near-black page is a different problem with a different answer.
+  - Light `--accent` is **not** the dark one, in hue or in step. Dark's `#0090ff` is 3.3:1 on white; Claude's brand terracotta `#cc785c` is 3.1:1 on ivory and 3.3:1 under white text, so it works as a large fill and fails as a link or a button label. Light mode uses it two steps down, `#a34a28` — 5.3:1 as text on a card, 5.9:1 white-on-accent.
+  - `--fg-muted` carries real content (timestamps, counts), so it is held at ≥4.5:1 (`#686359`, 5.4:1 on a card), not at the decorative value the Radix step suggests.
+  - Code colouring goes through the `syntax.*` tokens (`--syn-*`), never fixed Tailwind steps: the dark set is pastel (300-level), the light set is the same hues four steps darker, because a pastel keyword on a light code block is unreadable.
+  - Anything that assumes a dark background — a hardcoded near-black panel, `hover:text-white`, `filter: invert(1)` on a form control glyph — is a light-mode bug. Use tokens, or scope the rule under `.dark`.
+  - **A status/accent FILL is pre-mixed, never translucent.** `bg-status-success/10` and `border-status-danger/40` compile to `color-mix(… , rgb(var(--tint-base)))`, not to `rgb(… / 0.1)` — see `tint()` in `tailwind.config.js`. A translucent tint composites with whatever is behind it, so one "success" chip rendered as five different colours over the five surface steps and washed out on light cards. `--tint-base` is the theme's card surface, so the chip reads the same on the page, in a card, in a modal and inside an input row. This is scoped to `backgroundColor`/`borderColor`: `text-status-warning/80` and `ring-accent/40` stay genuinely translucent (they sit *on* a fill), and the `surface` family keeps its alpha or every scrim would turn solid. Light `--status-*` is two steps darker than Radix 11 so a label clears 4.5:1 against its own densest chip (a 20% tint of itself), not just against the card.
 - **React Router 6.28** - Client-side routing with nested layouts
 - **WebSocket** - Real-time event streaming from server
 - **Lucide Icons** - Modern, consistent icon set