feat(dashboard): expand themes to fonts, layout, density (#14725)

Dashboard themes now control typography and layout, not just colors.
Each built-in theme picks its own fonts, base size, radius, and density
so switching produces visible changes beyond hue.

Schema additions (per theme):

- typography — fontSans, fontMono, fontDisplay, fontUrl, baseSize,
  lineHeight, letterSpacing. fontUrl is injected as <link> on switch
  so Google/Bunny/self-hosted stylesheets all work.
- layout — radius (any CSS length) and density
  (compact | comfortable | spacious, multiplies Tailwind spacing).
- colorOverrides (optional) — pin individual shadcn tokens that would
  otherwise derive from the palette.

Built-in themes are now distinct beyond palette:

- default  — system stack, 15px, 0.5rem radius, comfortable
- midnight — Inter + JetBrains Mono, 14px, 0.75rem, comfortable
- ember    — Spectral (serif) + IBM Plex Mono, 15px, 0.25rem
- mono     — IBM Plex Sans + Mono, 13px, 0 radius, compact
- cyberpunk— Share Tech Mono everywhere, 14px, 0 radius, compact
- rose     — Fraunces (serif) + DM Mono, 16px, 1rem, spacious

Also fixes two bugs:

1. Custom user themes silently fell back to default. ThemeProvider
   only applied BUILTIN_THEMES[name], so YAML files in
   ~/.hermes/dashboard-themes/ showed in the picker but did nothing.
   Server now ships the full normalised definition; client applies it.
2. Docs documented a 21-token flat colors schema that never matched
   the code (applyPalette reads a 3-layer palette). Rewrote the
   Themes section against the actual shape.

Implementation:

- web/src/themes/types.ts: extend DashboardTheme with typography,
  layout, colorOverrides; ThemeListEntry carries optional definition.
- web/src/themes/presets.ts: 6 built-ins with distinct typography+layout.
- web/src/themes/context.tsx: applyTheme() writes palette+typography+
  layout+overrides as CSS vars, injects fontUrl stylesheet, fixes the
  fallback-to-default bug via resolveTheme(name).
- web/src/index.css: html/body/code read the new theme-font vars;
  --radius-sm/md/lg/xl derive from --theme-radius; --spacing scales
  with --theme-spacing-mul so Tailwind utilities shift with density.
- hermes_cli/web_server.py: _normalise_theme_definition() parses loose
  YAML (bare hex strings, partial blocks) into the canonical wire
  shape; /api/dashboard/themes ships full definitions for user themes.
- tests/hermes_cli/test_web_server.py: 16 new tests covering the
  normaliser and discovery (rejection cases, clamping, defaults).
- website/docs/user-guide/features/web-dashboard.md: rewrite Themes
  section with real schema, per-model tables, full YAML example.

web/src/lib/api.ts

@@ -1,5 +1,7 @@
 const BASE = "";
 
+import type { DashboardTheme } from "@/themes/types";
+
 // Ephemeral session token for protected endpoints.
 // Injected into index.html by the server — never fetched via API.
 declare global {
@@ -486,6 +488,9 @@ export interface DashboardThemeSummary {
   description: string;
   label: string;
   name: string;
+  /** Full theme definition for user themes; undefined for built-ins
+   *  (which the frontend already has locally). */
+  definition?: DashboardTheme;
 }
 
 export interface DashboardThemesResponse {