refactor(web): dashboard typography & contrast pass

Removes the global `uppercase` + `font-mondwest` from the App.tsx root
that forced every page to opt-out, replaces stacked-alpha text colors
with semantic tokens for WCAG-AA contrast across all 7 themes, and
applies the new `text-display` utility from @nous-research/ui@0.16.0
on intentional brand chrome (page titles, sidebar headings, segmented
filters) only. Bumps every sub-12px arbitrary text size to text-xs.

Also widens the dashboard plugin routes (/api/dashboard/agent-plugins/
{name:path}/...) so category-namespaced plugins like observability/
langfuse and image_gen/openai can be enable/disabled from the dashboard
— previously the FE encodeURIComponent-ed the slash and the backend
{name} route rejected it. _validate_plugin_name still blocks .. and
backslash, and strips leading/trailing slash.

Touches sessions/env/keys page chrome and adds two new i18n keys
(`overview`, `showMore`/`showLess`) across all 18 locales.

Squashes 19 commits from PR #28832.

Co-authored-by: Hermes <noreply@nousresearch.com>

web/src/index.css

@@ -146,7 +146,11 @@ code { font-size: 0.875rem; }
   --color-secondary: color-mix(in srgb, var(--midground-base) 6%, var(--background-base));
   --color-secondary-foreground: var(--midground);
   --color-muted: color-mix(in srgb, var(--midground-base) 8%, var(--background-base));
-  --color-muted-foreground: color-mix(in srgb, var(--midground-base) 55%, transparent);
+  /* Routes the shadcn `muted-foreground` slot through the DS semantic
+     text-secondary token (defaults to midground 80%) so legacy call
+     sites that use `text-muted-foreground` get a readable color
+     instead of the old 55%-transparent default. */
+  --color-muted-foreground: var(--color-text-secondary);
   --color-accent: color-mix(in srgb, var(--midground-base) 10%, var(--background-base));
   --color-accent-foreground: var(--midground);
   --color-destructive: #fb2c36;