Add Settings page with .env manager

- Sidebar: ⚙ Settings link pinned to bottom of nav
- Settings page: view all config keys (secrets masked), edit and save directly to .env
- Show/hide toggle for secret fields (tokens, keys)
- First-time setup banner with cp .env.example .env instructions when .env is missing
- Manual setup section with step-by-step terminal commands and where to find each credential
- New .env.example template with comments for all required variables
- Backend: GET/POST /api/settings/config router reads/writes mounted .env file
- docker-compose: mounts .env into backend container at /app/.env for write access

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,24 @@
+# ─────────────────────────────────────────────────────────────────────────────
+# SIEM Toolkit — Environment Configuration
+# ─────────────────────────────────────────────────────────────────────────────
+# 1. Copy this file:   cp .env.example .env
+# 2. Fill in values below (see comments for where to find each one)
+# 3. Start the app:    docker-compose up -d --build
+# ─────────────────────────────────────────────────────────────────────────────
+
+# SentinelOne Management Console
+# ─ URL: your console (e.g. https://demo.sentinelone.net)
+# ─ Token: Settings → Users → Service Users → generate API token
+S1_BASE_URL=https://demo.sentinelone.net
+S1_API_TOKEN=
+
+# Singularity Data Lake (SDL) — PowerQuery credentials
+# ─ Console: Settings → Integrations → Data Lake API Keys
+# ─ XDR URL: shown on the API Keys page (e.g. https://xdr.us1.sentinelone.net)
+# ─ Log Read Key: copy the "Log Read" key from that page
+SDL_XDR_URL=https://xdr.us1.sentinelone.net
+SDL_LOG_READ_KEY=
+
+# Anthropic (for Onboarding Accelerator AI features)
+# ─ https://console.anthropic.com/settings/api-keys
+ANTHROPIC_API_KEY=