Add first-login onboarding modal with markdown guide

Author: wilhel1812

docs/onboarding.md

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+# LinkSim Onboarding
+
+## 1. Scenarios
+- A **simulation** is your working setup: selected sites, links, and radio/model settings.
+- Open an existing simulation from **Simulation Library** or create a new one.
+- Use clear names so teammates can find simulations quickly.
+
+## 2. Sites
+- Sites are physical node locations with coordinates, ground elevation, and antenna height.
+- Manage your reusable site collection in **Site Library**.
+- Add sites by coordinates, map pick, search, or Meshtastic sources.
+- Add sites from the library into the current simulation as needed.
+
+## 3. Links
+- A link is a **From -> To** path between two sites used for path analysis.
+- Links let you compare multiple candidate paths in one simulation.
+- Start with the two sites you care about most, then add relay candidate links.
+
+## 4. Other Settings
+- **Channel**: frequency, bandwidth, SF, coding rate, TX power, gains, cable loss.
+- **Propagation model**: FSPL, TwoRay, or ITM (terrain-aware approximation).
+- **Terrain**: fetch and refresh terrain before relying on pass/fail decisions.
+- **RX target**: your decision threshold; pass/fail is based on this target.
+
+## 5. Map and Plots
+- The map shows channel/coverage overlays for the selected simulation and link.
+- Use map controls to fit, inspect terrain, and switch visualization modes.
+- The **path profile** shows terrain, LOS/Fresnel context, and endpoint relationship.
+- Use overlays to find potential relay positions and validate route quality.
+
+## 6. User Rights and Permissions
+- User roles: **Pending**, **User**, **Moderator**, **Admin**.
+- Resource visibility: **Private**, **Public**, **Shared**.
+- Collaborators grant edit rights on a site or simulation.
+- Moderation/admin actions are audited; use least privilege where possible.
+
+## Recommended Workflow
+1. Open or create a simulation.
+2. Add sites from Site Library.
+3. Create links for the paths you want to evaluate.
+4. Auto-fetch terrain data.
+5. Set channel + model, then inspect map + path profile.
+6. Save the simulation and share/collaborate as needed.