Add PWA install prompt for mobile devices

[Copilot]

Mobile users had no guidance on installing the SkyEmu PWA to their home screen. Added a modal popup that appears on first mobile visit with platform-specific installation instructions.

Implementation

Modal popup with device detection

• Detects mobile via user agent + navigator.maxTouchPoints > 0
• Platform-specific instructions for iOS Safari, Android Chrome, and generic mobile browsers
• Styled with SkyEmu theme colors (#759091)

User preference handling

• Shows 2s after page load on first mobile visit
• localStorage persistence for "Don't show again" option
• Suppressed when already running in standalone mode
• Event listeners attached once via flag to prevent duplicates

UI

Desktop (with generic instructions)

Mobile (Android-specific instructions)

Files Changed

• src/shell.html - Added popup HTML, CSS, and JavaScript logic

Original prompt

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This section details on the original issue you should resolve

<issue_title>Web build has no install button for PWA.</issue_title>
<issue_description>I heard this was a progressive web app so I thought i was able to install it, but i cant see the install app button on chrome or anywhere.</issue_description>

<agent_instructions>Can you add a popup with instructions or a link to add the PWA to the homescreen when first opened on a mobile device. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Web build has no install button for PWA. #543

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[github-actions]

Web build for c571fe9 will be live at https://web.skyemu.app/commit/c571fe98f7fafc24b11e4e4c9504296b81345768/index.html

[github-actions]

Web build for e78a89e will be live at https://web.skyemu.app/commit/e78a89e8e0f7ad54c6200207e7e545432effe574/index.html

[github-actions]

Web build for 8871e7a will be live at https://web.skyemu.app/commit/8871e7ab350f2ebaa640d2b047ff20b660f1a606/index.html