fix: always show page sidebar and read current page on play

[nathanael]

Summary

• Always show the page sidebar -- previously it only appeared when multiple pages existed, but the Add Page button lives inside the sidebar, making it impossible to add pages from a single-page state.
• Play reads the current page -- pressing play always reset to page 0 (the welcome text) instead of reading whichever page the user was currently editing. Now it reads the active page, falling back to the first non-empty page only if the current one is blank.

Test plan

• Open the app fresh -- sidebar should be visible with one page and Add Page button at the bottom
• Add a second page, type text, press play -- should read the page you are viewing, not page 1
• With only the welcome text on page 1, add a new page with custom text, select it, press play -- should read your custom text

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