Add DaxScaffold and lint rule enforcing it over Material Scaffold

[GerardPaligot]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1215496415658080/task/1215518884361562

Description

Adds a DuckDuckGo-themed Compose scaffold and a lint rule that enforces its use.

• DaxScaffold wraps Material3 Scaffold with DuckDuckGo default container/content colors and content window insets.
• Showcased with a top app bar (back + AI chat icon buttons) in the design system internal screen as a new "Scaffold" component.
• Adds the NoMaterial3ScaffoldUsage lint rule (error severity) that directs usage to DaxScaffold everywhere except the design system modules, mirroring the existing Material3 switch/checkbox/radio rules.

Stacked on top of #8823 — the scaffold's top bar uses DaxIconButton, which only themes correctly (dark mode) with that PR's color fix.

Steps to test this PR

Design system screen

• Build & install the internal variant, open the Design System → Layout.
• Scroll to "Scaffold" component: confirm the top app bar (title + back + AI chat icons) and content render.
• Toggle dark/light theme: confirm the scaffold background, top bar, and icons follow the theme.

UI changes

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Note

Low Risk
Changes are confined to new design-system API, internal demo UI, and lint enforcement with no production feature logic.

Overview
Introduces DaxScaffold, a design-system wrapper around Material3 Scaffold that applies DuckDuckGo theme container and content colors (via DaxScaffoldDefaults) while keeping the usual top/bottom bar, snackbar, insets, and content slot API.

The internal Design System → Layout catalog gains a Scaffold entry (Component.SCAFFOLD, layout + ScaffoldComponentViewHolder) demoing DaxSearchTopAppBar with search state over placeholder content.

A new NoMaterial3ScaffoldUsage lint check (error severity, registered in DuckDuckGoIssueRegistry) flags direct androidx.compose.material3.Scaffold calls outside design-system / design-system-internal, with unit tests matching the existing Material3 component lint pattern.

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[GerardPaligot]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Migrate AboutDuckDuckGoActivity from XML to Compose #8915
• Add DaxScaffold and lint rule enforcing it over Material Scaffold #8824 👈 (View in Graphite)
• Create DaxTopAppBar and DaxSearchTopAppBar in Compose #8904
• Theme DaxIconButton colors and add filled variant #8823
• develop

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