[Design P2] Fix layout consistency: main dashboard and network page must use DashboardLayout

[AnilChinchawaleXDC]

Source: DESIGN-AUDIT.md

Problem: Main dashboard and Network page use Sidebar directly instead of DashboardLayout, creating inconsistency.

Actions:

1. Migrate main dashboard to use DashboardLayout
2. Migrate network page to use DashboardLayout
3. Remove direct Sidebar usage where layout component should be used
4. Verify no visual regressions

Risk: Inconsistent UX, maintenance overhead.

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Mirrored from Paperclip OPEA-368 (https://agi.openscan.ai/OPEA/issues/OPEA-368)