feat: add first-launch walkthrough and smooth exercise start

[jiaxinnns]

Overview

New learners had no orientation and Start/Verify gave little feedback. This adds a two-step walkthrough and tightens the exercise start flow, tour nav, and AI hint model choice.

Solution

• Walkthrough on first launch (and via a Help button): dims the terminal to point at lessons, then dims the lesson page to point at the terminal. The lesson page is dimmed by injecting an overlay into it, since DOM can't cover the native view
• Start Exercise / Verify show a spinner and are disabled until main finishes; every successful start shows a "Started exercise" toast
• Start intro modal repeats until "Don't show this again" is checked
• Tour rows only expand/collapse; each tour gets a Tour Home link; tours panel open by default
• Window maximises on launch
• AI hints use OpenRouter's free router instead of three named models

Test cases

• Clear gm-walkthrough-seen from localStorage and relaunch: the walkthrough shows, and Next/Back/Done work in light and dark mode
• Help button in the header restarts the walkthrough
• Click Start Exercise / Verify: button disables with a spinner until done
• Start an already downloaded exercise: "Started exercise" toast still appears
• Intro modal shows on each Start until the checkbox is ticked, then stops
• Clicking a tour row only toggles it; Tour Home opens the tour page
• App opens maximised
• Request an AI hint: it returns using openrouter/free

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