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/index.css

@@ -29,6 +29,48 @@
   /* Consumed by <Backdrop />; also theme-switchable. */
   --warm-glow: rgba(255, 189, 56, 0.35);
   --noise-opacity-mul: 1;
+
+  /* Typography tokens — rewritten by ThemeProvider. Defaults match the
+     system stack so themes that don't override look native. */
+  --theme-font-sans: system-ui, -apple-system, "Segoe UI", Roboto,
+    "Helvetica Neue", Arial, sans-serif;
+  --theme-font-mono: ui-monospace, "SF Mono", "Cascadia Mono", Menlo,
+    Consolas, monospace;
+  --theme-font-display: var(--theme-font-sans);
+  --theme-base-size: 15px;
+  --theme-line-height: 1.55;
+  --theme-letter-spacing: 0;
+
+  /* Layout tokens. */
+  --radius: 0.5rem;
+  --theme-radius: 0.5rem;
+  --theme-spacing-mul: 1;
+  --theme-density: comfortable;
+}
+
+/* Theme tokens cascade into the document root so every descendant inherits
+   the font stack, base size, and letter spacing without explicit calls. */
+html {
+  font-family: var(--theme-font-sans);
+  font-size: var(--theme-base-size);
+  line-height: var(--theme-line-height);
+  letter-spacing: var(--theme-letter-spacing);
+}
+
+body {
+  font-family: var(--theme-font-sans);
+}
+
+code, kbd, pre, samp, .font-mono, .font-mono-ui {
+  font-family: var(--theme-font-mono);
+}
+
+/* Density: scale the shadcn spacing utilities via a multiplier. The DS
+   components use `p-N` / `gap-N` / `space-*` classes which resolve against
+   Tailwind's spacing scale; multiplying `--spacing` at :root scales them
+   all proportionally in Tailwind v4. */
+@theme inline {
+  --spacing: calc(0.25rem * var(--theme-spacing-mul, 1));
 }
 
 /* Nousnet's hermes-agent layout bumps `small` and `code` to readable
@@ -65,6 +107,11 @@ code { font-size: 0.875rem; }
   --color-ring: var(--midground);
   --color-popover: color-mix(in srgb, var(--midground-base) 4%, var(--background-base));
   --color-popover-foreground: var(--midground);
+
+  --radius-sm: calc(var(--theme-radius) - 4px);
+  --radius-md: calc(var(--theme-radius) - 2px);
+  --radius-lg: var(--theme-radius);
+  --radius-xl: calc(var(--theme-radius) + 4px);
 }
 
 
@@ -94,9 +141,11 @@ code { font-size: 0.875rem; }
 
 /* System UI-monospace stack — distinct from `font-courier` (Courier
    Prime), used for dense data readouts where the display font would
-   break the grid. */
+   break the grid. Routes through the theme's mono stack so themes
+   with a different monospace (JetBrains Mono, IBM Plex Mono, etc.)
+   still apply here. */
 .font-mono-ui {
-  font-family: ui-monospace, 'SF Mono', 'Cascadia Mono', Menlo, monospace;
+  font-family: var(--theme-font-mono);
 }
 
 /* Subtle grain overlay for badges. */