fix: add fallback download URLs and mid-stream retry for model downloads

[dcondrey]

Summary

• The primary download server (meetily.towardsgeneralintelligence.com) is failing TLS handshakes, causing all first-run model downloads to fail during onboarding for new users
• Adds HuggingFace fallback URLs for both Parakeet transcription and Gemma summary models
• Restructures download logic so mid-stream failures (timeout, dropped connection) also trigger fallback instead of just erroring out
• Fixes a bug where the 416 Range Not Satisfiable retry path used the wrong HTTP response for streaming

Changes

parakeet_engine.rs — Parakeet transcription model download:

• base_url (single) → base_urls (Vec) with HuggingFace fallback (istupakov/parakeet-tdt-0.6b-v3-onnx)
• Entire download (connect + stream) wrapped in labeled 'url_loop — mid-stream failures continue to next URL with resume
• Re-checks existing_size before each URL attempt so resume works after partial downloads
• Fixes bug: 416 retry now properly reassigns resp so streaming reads from the correct response

model_manager.rs — Gemma summary model download:

• Same 'url_loop restructure wrapping connection + streaming
• Mid-stream timeout/error flushes partial file and tries next URL with resume
• Better error messages: "all download servers failed" instead of "check your internet"

models.rs — Model definitions:

• Added fallback_urls: Vec<String> field to ModelDef (with #[serde(default)])
• Gemma 1B fallback: ggml-org/gemma-3-1b-it-GGUF (verified public, no auth)
• Gemma 4B fallback: ggml-org/gemma-3-4b-it-GGUF (verified public, no auth)

Verified fallback URLs

Model	Fallback URL	Status
Parakeet v3	huggingface.co/istupakov/parakeet-tdt-0.6b-v3-onnx	307 ✓
Gemma 1B	huggingface.co/ggml-org/gemma-3-1b-it-GGUF	302 ✓
Gemma 4B	huggingface.co/ggml-org/gemma-3-4b-it-GGUF	302 ✓

Related issues

• Relates to [BUG] Gemma3:4b Unabled to load while generating summary #405 — Gemma 4B "unable to load" likely caused by corrupted download from failing server
• Relates to [BUG] Transcription model not ready #318 — Transcription model not ready; more reliable downloads reduce how often users hit this

Test plan

• Clean install: delete model directories, run app, verify onboarding downloads succeed via fallback
• Partial download resume: interrupt download mid-stream, restart, verify it resumes correctly
• Verify both Parakeet and Gemma models download and load successfully after onboarding
• Verify retry button works when download fails