feat: add floating help chat widget, remove settings page

Add a documentation-grounded help chat assistant accessible from every
page via a floating button in the bottom-right corner.

Backend (compliance-agent):
- New POST /api/v1/help/chat endpoint
- Loads README.md + docs/**/*.md at first request (OnceLock cache)
- Excludes node_modules, uses walkdir for discovery
- Falls back to degraded prompt if docs not found
- Uses LiteLLM via existing chat_with_messages infrastructure

Dashboard (compliance-dashboard):
- New HelpChat component with toggle button, message area, input
- Styled to match Obsidian Control theme (dark, accent cyan)
- Renders in AppShell so it's available on every page
- Multi-turn conversation with history
- Server function proxies to agent API

Also:
- Remove Settings page (route, sidebar entry, page file)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

compliance-dashboard/src/components/sidebar.rs

@@ -52,11 +52,6 @@ pub fn Sidebar() -> Element {
             route: Route::PentestDashboardPage {},
             icon: rsx! { Icon { icon: BsLightningCharge, width: 18, height: 18 } },
         },
-        NavItem {
-            label: "Settings",
-            route: Route::SettingsPage {},
-            icon: rsx! { Icon { icon: BsGear, width: 18, height: 18 } },
-        },
     ];
 
     let docs_url = option_env!("DOCS_URL").unwrap_or("/docs");