feat(dashboard): live theme preview in ThemeSelectorModal

When the modal is open, the targeted component re-renders with the
candidate theme as soon as the user picks an option — no dashboard-
dirty round-trip required. Cancel reverts; Apply commits to Redux.

Implementation:

- New \`previewThemeStore\` (module-level subscribable map keyed by
  layoutId, distinguishing "no preview" / "preview value=null" /
  "preview value=number"). Tiny surface: \`set\`/\`clear\`/\`get\`/
  \`subscribe\`. No-op \`set\` / \`clear\` calls don't fire listeners.
- \`useEffectiveThemeId\` now subscribes via \`useSyncExternalStore\`
  and prefers the preview value over the Redux-resolved id when
  present.
- \`ThemeSelectorModal\` writes the in-flight selection through the
  store as the user picks options; cleanup on close (Cancel, X
  button, escape) clears it. Apply dispatches the Redux action
  *before* hiding, so the post-cleanup re-resolution lands on the
  saved value (no flicker).

Snapshot of the resolved id at open-time goes through a \`useRef\`
because \`currentThemeId\` itself becomes reactive (it would already
reflect the in-flight preview), so we can't read it for "what should
we revert to?".

6 new tests for the preview store: get-undefined-for-unknown,
set-stores-numeric, set-stores-explicit-null, clear-removes,
subscriber-fires-on-real-change-and-not-on-no-op, multi-layoutId-
independence. Total dashboard ComponentThemeProvider suite is now
14 passing tests.

Drops "live preview" from the deferred-items list in SIP.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>