feat(inbox): Two-pane layout (#1408)

## Problem

Code inbox uses a right sidebar that opens/closes and replaces the whole view for empty states. Cloud has a permanent two-pane split more like an email app, with empty states shown contextually. Let's go for the more robust email-like experience.

## Changes

Adding an always-visible resizable left pane (report list) + right pane (detail) when reports exist.

When no reports: full-width view with toolbar + faded skeleton backdrop, gradient overlay, and contextual empty state (welcome with graphs-hog, warming-up with explorer-hog). When reports exist but none selected: mail-hog "Select a report" prompt in right pane.

Signal source config is in a modal instead instead of navigating away to Settings.

Search disabled with tooltip when project has no reports.

Author: Twixes