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/plugins/PluginPage.tsx

@@ -35,7 +35,7 @@ export function PluginPage({ name }: { name: string }) {
       <div
         className={cn(
           "max-w-lg p-4",
-          "font-mondwest text-sm tracking-[0.08em] text-midground/80",
+          "font-mondwest text-sm tracking-[0.08em] text-text-secondary",
         )}
         role="alert"
       >
@@ -48,7 +48,7 @@ export function PluginPage({ name }: { name: string }) {
     <div
       className={cn(
         "flex items-center gap-2 p-4",
-        "font-mondwest text-sm tracking-[0.1em] text-midground/60",
+        "font-mondwest text-sm tracking-[0.1em] text-text-tertiary",
       )}
     >
       <Spinner className="shrink-0" />