feat: implement SidePanel component

[patricklx]

Summary

• Implements SidePanel, previously missing from the Ember addon (carbon-components-ember/src/components/side-panel.gts), covering the full React prop surface: actions, actionToolbarButtons (leading/icon-only variants), currentStep/onNavigationBack, includeOverlay/preventCloseOnClickOutside, slideIn/selectorPageContent, resizable, decorator (plus deprecated slug/aiLabel handled the same way React treats them — as icon aliases, via the icon/renderIcon alias on toolbar buttons), selectorPrimaryFocus, launcherButtonRef (as a plain HTMLElement rather than React's RefObject, since Ember has no ref objects), animateTitle, and size/placement.
• Translates React's hooks/refs into this addon's existing idioms: functional ember-modifiers for DOM work (presence/exit-animation tracking, focus-on-open, Escape-to-close, resize drag/keyboard handling, and the scroll-driven title collapse), a ComponentLike arg for @decorator/toolbar icons, and a subtitle block for rich content alongside the plain @subtitle string arg.
• The scroll-driven title-collapse effect is implemented by setting the same CSS custom property (--cds--side-panel--scroll-animation-progress) Carbon's own stylesheet already keys off, rather than re-implementing React's per-element margin bookkeeping.
• Bumps @carbon/styles from 1.112.0 to 1.113.0 (already permitted by the existing ^1.112.0 range) — 1.113.0 is the first release whose compiled CSS actually includes .cds--side-panel/.cds--resizer styles; without it the component would render unstyled.
• ActionSet/ButtonSet/Resizer/IconButton are not implemented as separate public components here (they're tracked separately, e.g. ButtonSet/IconButton are already open parity issues) — the footer action-button stacking/ordering and the resize handle are implemented privately inside side-panel.gts instead, matching their real Carbon CSS classes.
• Adds docs at docs-app/app/templates/2-components/side-panel.md with one live-preview example per React Storybook story (default/slide-over, slide-in, action toolbar, multi-step, decorator, initial-focus selector, static title, no-title, plus resizable).
• Adds tests at test-app/tests/components/side-panel-test.gts covering rendering, open/close, actions, toolbar buttons, decorator, overlay/outside-click, Escape, slide-in, and resizing.

Known simplifications (documented on the issue)

• The exit animation is driven by a small presence-tracking modifier (listens for animationend, deferred via ember-lifeline's runTask to avoid Ember's backtracking-rerender assertion) rather than React's usePresence hook — same visible behavior, different plumbing.
• usePrefersReducedMotion detection isn't replicated; Carbon's own @media (prefers-reduced-motion) CSS still applies.

Test plan

• pnpm build succeeds in carbon-components-ember/
• pnpm test -- --filter="SidePanel" in test-app/ — all 19 SidePanel tests pass (533/533 overall, no regressions)
• glint --declaration type-checks cleanly

Closes #748