feat(playground)!: redesign sidebar with nav-based layout and theme editing

[effie-ms]

NB:
To be reviewed after: #745 and #746 (isolated fixes for @lifi/widget)

Which Linear task is linked to this PR?

https://linear.app/lifi-linear/issue/EMB-348/restyle-widget-playground

Why was it implemented this way?

Widget changes

These affect the published @lifi/widget package.

• drop-shadow → box-shadow in createTheme.ts and watermelonLight.ts — updated to support spread control (to make it configurable from the playground sidebar)
• Grey palette standardized: grey[200]/grey[700] → grey[300]/grey[800] in SendToWalletPage.style.tsx (other grey pairs are not used within the widget)
• MuiDrawer added to allowed theme component overrides, now mergeable via createTheme.ts - to be able to customize Drawer variant
• Header spacing: removed header bottom padding, added equivalent top padding to content area in AppLayout - not to cut shadow.

Playground changes

• Complete sidebar redesign: old DrawerControls + ExpandableCard accordion → floating sidebar with SidebarView nav + slide-in detail views
• Removed @monaco-editor/react and @mui/lab dependencies - not supporting code editor anymore
• Store v1→v2: viewport color split per mode, code control/skeleton side-by-side removed, widget event monitors added
• withFloatingDrawer() wrapper isolates playground drawer overrides from widget themes

Checklist before requesting a review

• I have performed a self-review and testing of my code.
• This pull request is focused and addresses a single problem.

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