Download refactor

[gabrielste1n]

Summary

• Consolidated three duplicate download implementations (whisper.js, parakeet.js, modelManagerBridge.js) into a shared downloadUtils.js module, removing ~650 lines of redundant code
• Added download resume support — partial .tmp files are detected and resumed via HTTP Range headers, with automatic fallback if the server doesn't support Range
• Added retry with exponential backoff — transient network errors (ECONNRESET, ETIMEDOUT, etc.) are retried up to 3 times with capped backoff
• Replaced flag-based cancellation with abort signal pattern — new createDownloadSignal() provides a clean, reusable abort mechanism used by all three download paths
• Added "Installing" UI state for Parakeet — after download completes, the progress bar shows "Installing [model]..." with a pulsing animation during tar.bz2 extraction, instead of appearing stuck at 100%
• Added client-side progress throttle for whisper/parakeet — matches the existing LLM handler throttle to prevent UI flashing from high-frequency IPC updates
• Simplified IPC handlers — removed redundant try/catch wrappers in ipcHandlers.js that duplicated error/completion events; errors now propagate naturally as rejected IPC promises
• Extracted LocalModelCard to a standalone function component — moved out of the render body in TranscriptionModelPicker to avoid re-creation on every render
• Pre-resolves redirects — HEAD request resolves CDN redirect chains once before downloading, avoiding per-chunk redirect overhead
• Atomic file writes — downloads write to .tmp then rename, with EXDEV cross-device fallback for Windows/Linux edge cases