Add drop-in PWA install component with custom trigger and iOS fallback

[sbsoftware-agent]

Summary

This adds a pluggable Crumble::PwaInstallComponent that works by calling #to_html from a consuming layout and requires no extra integration.

Changes

• Added src/pwa_install_component.cr:
• ToHtml markup for a fixed-bottom mobile install CTA and iOS fallback instruction panel.
• Inline component CSS for required slim full-width bottom placement, slight yellow container, and grey button styling.
• JS::Code behavior fragment that:
• captures beforeinstallprompt and defers native prompting,
• triggers prompt() only from the custom button,
• clears deferred prompt state after user choice,
• handles accepted/dismissed outcomes,
• hides UI in standalone/installed mode,
• keeps CTA available on iOS Safari and opens fallback instructions there,
• supports close button and tap-outside-to-close for the fallback panel.
• Updated src/crumble.cr to require the new component.
• Added specs in spec/pwa_install_component_spec.cr covering:
• drop-in rendering via a consuming layout calling #to_html,
• deferred install prompt wiring and outcome handling,
• standalone/mobile gating and iOS Safari fallback behavior.

Verification

• CRYSTAL_CACHE_DIR=/tmp/crystal-cache-cr crystal spec

[sbsoftware]

Please use the js.cr DSL as much as possible, and only fall back to _literal_js for those instructions that aren't supported.
Also use the css.cr DSL with css_classes instead of custom attributes.

[sbsoftware-agent]

Implemented in commit f08d536.

Changes made:

• Rewrote Crumble::PwaInstallComponent::Script to use js.cr DSL end-to-end (def_to_js with DSL control flow/callbacks), removing the previous large _literal_js block.
• Replaced custom id-driven styling/hooks with css.cr classes:
  • added css_classes for container/button/panel/dialog/text/close button,
  • added PwaInstallComponent::Style < CSS::Stylesheet with rule/media DSL,
  • removed inline raw <style> CSS.
• Updated template markup to use those CSS classes and include Style + Script as component children.
• Updated spec/pwa_install_component_spec.cr to validate class-based markup, stylesheet asset output, and updated JS emission.

Verification:

• CRYSTAL_CACHE_DIR=/tmp/crystal-cache-cr crystal spec spec/pwa_install_component_spec.cr
• CRYSTAL_CACHE_DIR=/tmp/crystal-cache-cr crystal spec

Both passed.