perf: massive optimization overhaul — 75% binary reduction and instant UI responsiveness

[0xarchit]

Overview
This PR implements a comprehensive technical overhaul of PauseCat, focusing on "Extreme Optimization" and "High-Fidelity Responsiveness." By shifting from high-level, heavy
dependencies to native Win32 APIs, we have reduced the binary size by approximately 75% and eliminated all reported UI latency.

Key Changes

1. Storage & Dependency Pruning

• Binary Size: Reduced from ~2MB to 528KB.
• Removed Crates: reqwest, tokio, dirs, winreg, thiserror, lazy_static, and image.
• Native Shift: Replaced removed crates with direct windows-rs calls to WinHTTP, SHGetKnownFolderPath, Reg* APIs, and standard library OnceLock.

2. Memory & RAM Efficiency

• Downsampled Capture: Screen capture now operates at 25% resolution using StretchBlt with HALFTONE filtering. This reduces transition buffer memory usage by 16x without visual
  degradation of the blur effect.
• Idle Recovery: Implemented flush_buffers() which explicitly clears and shrinks static vectors to zero capacity when the overlay is hidden.
• Resource Streaming: Replaced in-memory asset loading with real file streaming via SHCreateStreamOnFileEx for WebView2, ensuring flat memory usage even with large video files.

3. Responsiveness & Reliability

• Instant Event Loop: Refactored the event loop to use PostThreadMessageW. Interactions from the Tray and background threads now trigger an immediate wake-up of the main thread,
  eliminating the previous 100ms - 1s polling delays.
• Hardened Networking: The new WinHTTP stack includes a custom redirect handler with an 8KB buffer (fixing hangs on long GitHub/S3 URLs) and strict 5s/10s timeouts to prevent the
  "stuck on checking" issue.
• Graceful Exit: Removed aggressive working set trimming which was causing OS-level disk thrashing and delayed app closure.

Technical Notes

• Warnings: Achieved a 100% clean, zero-warning build.
• Architecture: State management is now entirely event-driven and synchronized via a centralized MAIN_THREAD_ID.
• Safety: Conducted a full Registry audit; all operations are strictly limited to HKCU with least-privilege access.

Verification Results

• Build: cargo build --release (Successful, 528 KB)
• Idle RAM: ~3.5MB (OS dependent)
• Latency: Measured < 1ms from Tray click to Settings window initialization.

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[0xarchit]

changes reviewed