fix(shell): nonce inline theme-restore script and code style under a nonce-based CSP

[mhenrixon]

Problem

DocsUI::Shell emits an inline <script> (the theme-restore script) and DocsUI::Code emits an inline <style> — neither carried a CSP nonce. On a host app that enforces a nonce-based script-src (Rails' default when content_security_policy_nonce_directives includes script-src), the browser blocks these inline tags. The visible casualty: the persisted theme never applied on first paint.

Fix

• DocsUI::Shell and DocsUI::Code now include Phlex::Rails::Helpers::ContentSecurityPolicyNonce and emit their inline tag with nonce: from a small private #csp_nonce.
• #csp_nonce guards on view_context before delegating to the phlex-rails value helper. In phlex-rails 2.4.0 the content_security_policy_nonce helper delegates to the view context — which is nil in an isolated render (or on a host that doesn't nonce script-src) and would otherwise raise — so we return nil there. Phlex omits a nil-valued attribute, so the un-nonced markup stays byte-identical to before.
• style-src often still allows 'unsafe-inline', but Code's inline <style> is nonced too, for symmetry and future-proofing.

Graceful degradation is the whole point: on a host without a nonce, nothing changes; on a host with one, the tag carries it and the browser allows it.

Test plan

• spec/docs_ui/shell_spec.rb (new): the theme-restore <script> renders the JS; with no nonce it carries no nonce attribute; with a nonce it renders nonce="testnonce". Plus a focused proof that Phlex omits a nil-valued attribute (the primitive the fix relies on).
• spec/docs_ui/code_spec.rb: the inline <style> carries no nonce with no view context and nonce="testnonce" when a nonce is present.
• spec/spec_helper.rb loads phlex-rails (plus the two ActiveSupport core-exts it and the theme-restore script need) so DocsUI:: components that compose phlex-rails helpers render standalone in the suite.

Verification

• bundle exec rspec — 55 examples, 0 failures (stable across seeds 1, 42, 999, and random).
• bundle exec rubocop app lib spec — 31 files, no offenses.
• No-nonce path is byte-identical: no nonce attribute is emitted when there is no CSP nonce, so existing consuming sites and all prior specs are unaffected.

Closes #2