Replace post-signup modal with a single guided empty state

Description: The first screen after signup currently presents three paths — use a template, start from scratch, or just get an API key — with instructions for each written as text in a modal. This asks developers to make a decision before they have enough context to make it confidently. The "just create an API key" option in particular is not a meaningful user goal.

Proposed change: Replace the three-path modal with a single guided empty state that points the developer toward one clear next step based on where they are. Add a specific docs link directly in the dashboard so developers don't have to manually comb through the sidebar to find the right starting point.

Why this matters: On a first attempt following the "just create an API key" path, getting set up locally took approximately 10 minutes due to uncertainty about what to do next. A single guided path with a direct docs link eliminates that ambiguity.

@MichaelMilstead.