Fix viewport meta tag, seek() float precision, and add open issues summary

[Copilot]

Three long-standing issues addressed: missing viewport meta tag causing small controls on device, seek() truncating sub-second positions due to Int parameter, and no consolidated view of open issues for contributors.

Changes

• player.html — Added <meta name="viewport" content="width=device-width, initial-scale=1.0">. Without it, WKWebView uses desktop layout width, making player controls tiny on non-retina/variable-DPI devices.

• YTSwiftyPlayer.swift — Changed seek(to:allowSeekAhead:) parameter from Int to Double. The IFrame API's seekTo() accepts fractional seconds; the Int constraint was silently flooring positions like 30.5s → 30s. Existing integer call sites promote automatically.

// Before
player.seek(to: 30, allowSeekAhead: true)

// After — sub-second precision now works
player.seek(to: 30.5, allowSeekAhead: true)

• README.md — Added a Known Issues table listing all open issues with links and categories (Bug / Enhancement / Question) to help contributors triage.

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