Implement Initial Prompt w/ Intro

[PauloMFJ]

Overview

Adds the home-screen entry point for the Atlas: a prompt bar and an initial-mount welcome overlay above it.

• Prompt (scopes/page_home/prompt.tsx) — auto-growing textarea (field-sizing, capped at 4 rows) with a visually-hidden label for a11y, Enter-to-submit (Shift+Enter for newline), and a send Button.
• Intro overlay (scopes/page_home/intro.tsx) — dismissible first-run card laid out with CSS grid (close button in its own column), an intro blurb, clickable example prompts, and a "progress isn't saved" notice. Picking an example adds the query to the prompt - it can then be manually submitted.
• page_home promoted to a folder and now composes the prompt + intro, owning the query/intro state.
• Supporting: new arrow_up / close / info icons; _reset.scss now neutralizes textarea; new typography mixins (type-display, type-body-large, type-input); --shadow-elevated + button-size CSS vars in _root.scss; added card-base-subtle token.

[gemini-code-assist]

Code Review

This pull request implements a visual canvas exploration interface using tldraw, adding custom canvas controls, tools, an introductory screen with example prompts, and a prompt input component. The review feedback highlights several critical usability and robustness improvements, including handling IME composition in the prompt input to prevent premature submission, clearing active timeouts during rapid mock query submissions, preventing division-by-zero in the mapRange utility, and adding a browser fallback for the modern field-sizing CSS property. Additionally, feedback was provided to fix the animation transition on the intro screen, secure the tool name type guard against prototype inheritance, and ensure that selecting an example prompt correctly triggers the mock query.

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