fix: layout overflow regression + adopt true neutral gray theme

Problem: two issues surfaced after the dashboard-01 shell change
(cbfd09c). (1) Sidebar.Inset previously had an explicit h-svh that
hard-capped the app's height at the viewport; adding variant="inset"
put a margin on that same fixed-height box, pushing it taller than the
viewport with nothing left in the chain to cap it (Sidebar.Provider's
own wrapper only sets min-h-svh — a floor, not a ceiling). Result: the
whole page scrolled as one long document instead of each page's own
content scrolling internally with the header pinned — confirmed via
computed styles, e.g. Entities.svelte's table wrapper measured
scrollHeight 6531px against a 900px viewport, all of it spilling past
body instead of scrolling in its own rounded-border container.
(2) The color palette was GitHub-dark-inspired (blue-tinted grays:
#0d1117 bg, #58a6ff primary/accent) rather than the neutral grays the
shadcn-svelte dashboard-01 reference actually uses.

Change:
- App.svelte: moved the height cap up to Sidebar.Provider itself
  (class="h-svh") instead of Sidebar.Inset, since the cap needs to sit
  above wherever the inset variant's margin gets applied, not on the
  same box as the margin.
- app.css: replaced the core tokens (background/foreground/card/
  popover/primary/secondary/muted/accent/border/input/ring/sidebar-*)
  with shadcn's canonical dark-theme OKLCH values (0-chroma neutral
  grays), pulled directly from huntabyte/shadcn-svelte's own
  docs/src/app.css rather than approximated. --success/--warning
  deliberately kept as real, distinguishable colors — they signal
  actual health state, and desaturating them to match the neutral
  chrome would reintroduce the "can't tell what's actually happening"
  problem this whole project started from (see 279549c). --accent-blue
  now aliases --sidebar-primary (still a real blue) instead of
  --primary, so the couple of spots wanting an interactive "pop" still
  have one while buttons/links/focus rings ride the neutral --primary.

Risk: reversible_low (UI-only).

Verification: npx tsc --noEmit clean (excluding pre-existing unrelated
.svelte type-resolution warnings). go build/vet clean (backend
untouched, sanity check only). Manually verified in the browser
preview at 1400px: document.body.scrollHeight now exactly matches
window.innerHeight on both Overview and the 193-row Entities table
(previously 6531px vs 900px); scrolled the Entities table wrapper to
row ~60 and confirmed the header/filter bar/column headers stay
pinned while only the table body scrolls; confirmed neutral gray
rendering across Overview's stat cards, the event-rate chart, and
Chat's tool-call list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/App.svelte

@@ -77,7 +77,7 @@
 
 <Toaster />
 
-<Sidebar.Provider style="--header-height: calc(var(--spacing) * 12);">
+<Sidebar.Provider class="h-svh" style="--header-height: calc(var(--spacing) * 12);">
   <Sidebar.Root collapsible="icon" variant="inset">
     <Sidebar.Header>
       <Sidebar.Menu>
@@ -160,7 +160,7 @@
     </Sidebar.Footer>
   </Sidebar.Root>
 
-  <Sidebar.Inset class="h-svh min-h-0">
+  <Sidebar.Inset class="min-h-0 overflow-hidden">
     <header class="flex h-(--header-height) shrink-0 items-center gap-1 border-b px-4 lg:gap-2 lg:px-6">
       <Sidebar.Trigger class="-ms-1" />
       <Separator orientation="vertical" class="mx-2 data-[orientation=vertical]:h-4" />