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/components/LanguageSwitcher.tsx

@@ -69,12 +69,12 @@ export function LanguageSwitcher({ dropUp = false }: LanguageSwitcherProps) {
         aria-label={t.language.switchTo}
         aria-haspopup="listbox"
         aria-expanded={open}
-        className="px-2 py-1 normal-case tracking-normal font-normal text-xs text-muted-foreground hover:text-foreground"
+        className="px-2 py-1 normal-case tracking-normal font-normal text-xs text-text-secondary hover:text-foreground"
       >
         <span className="inline-flex items-center gap-1.5">
           <Typography
             mondwest
-            className="hidden sm:inline tracking-wide uppercase text-[0.65rem]"
+            className="hidden sm:inline text-display tracking-wide text-xs"
           >
             {locale === "en" ? "EN" : current.name}
           </Typography>