Feat: improve downloads of models under unstable connections

[stdevPavelmc]

This fixes #520 with some improvements

• Add CURL resume support for partial downloads
• Track progress correctly when resuming existing files
• Increase retry attempts and use exponential backoff
• Add remote file size lookup for resume logic
• Refactor pull flow so partial and missing files are handled together
• Remove redundant pre-download checks from main flow

Example of a session (this session was started and interrupted before several times to test and make an example of the features, it has 1 missing file and 2 partial downloads:

pavel@agatha:~$ flm pull qwen3.5:9b
[FLM]  Model: qwen3.5:9b
[FLM]  Name: Qwen3.5-9B-NPU2
[FLM]  Missing files (1):
  - chat_template.jinja
[FLM]  Present files (5):
  - config.json
  - model.q4nx
  - tokenizer.json
  - tokenizer_config.json
  - vision_weight.q4nx
[FLM]  Checking all files for missing or partial downloads...
[FLM]  Partial download detected: model.q4nx (374 MB / 7534 MB)
[FLM]  Partial download detected: vision_weight.q4nx (512 MB / 980 MB)
[FLM]  Downloading 3 file(s)...
[FLM]  Files to download (7627.10 MB): 
  - model.q4nx (7159.44 MB)
  - vision_weight.q4nx (467.66 MB)
  - chat_template.jinja (0.01 MB)
[FLM]  Downloading 1/3: model.q4nx
[FLM]  Resuming from 374.641 MB
[FLM]  Downloading: 21.4% (1611.6MB / 7534.1MB)CURL error: Transferred a partial file
[FLM]  Download failed (attempt 1/10)
[FLM]  Retrying in 1s...
[FLM]  Resuming from 1611.89 MB
[FLM]  Downloading: 37.2% (2801.0MB / 7534.1MB)CURL error: Failure when receiving data from the peer
[FLM]  Download failed (attempt 2/10)
[FLM]  Retrying in 2s...
[FLM]  Resuming from 2801.03 MB
[FLM]  Downloading: 54.7% (4121.2MB / 7534.1MB)CURL error: Failure when receiving data from the peer
[FLM]  Download failed (attempt 3/10)
[FLM]  Retrying in 4s...
[FLM]  Resuming from 4121.51 MB
[FLM]  Downloading: 54.8% (4125.7MB / 7534.1MB)CURL error: Failure when receiving data from the peer
[FLM]  Download failed (attempt 4/10)
[FLM]  Retrying in 8s...
[FLM]  Resuming from 4125.72 MB
[FLM]  Downloading: 54.8% (4130.8MB / 7534.1MB)CURL error: Failure when receiving data from the peer
[FLM]  Download failed (attempt 5/10)
[FLM]  Retrying in 16s...
[FLM]  Resuming from 4131.05 MB
[FLM]  Downloading: 54.8% (4131.7MB / 7534.1MB)CURL error: Failure when receiving data from the peer
[FLM]  Download failed (attempt 6/10)
[FLM]  Retrying in 30s...
[FLM]  Resuming from 4131.85 MB
[FLM]  Downloading: 100.0% (7533.7MB / 7534.1MB)
[FLM]  Checking Hash...
[FLM]  Download completed: /home/pavel/.config/flm/models/Qwen3.5-9B-NPU2/model.q4nx
[FLM]  Overall progress:  1/3 files
[FLM]  Downloading 2/3: vision_weight.q4nx
[FLM]  Resuming from 512.469 MB
[FLM]  Downloading: 99.9% (979.6MB / 980.1MB)
[FLM]  Checking Hash...
[FLM]  Download completed: /home/pavel/.config/flm/models/Qwen3.5-9B-NPU2/vision_weight.q4nx
[FLM]  Overall progress:  2/3 files
[FLM]  Downloading 3/3: chat_template.jinja
[FLM]  Downloading: 100.0% (0.0MB / 0.0MB)
[FLM]  Checking Hash...
[FLM]  Download completed: /home/pavel/.config/flm/models/Qwen3.5-9B-NPU2/chat_template.jinja
[FLM]  Overall progress:  3/3 files
[FLM]  All downloads completed successfully!
[FLM]  Model downloaded successfully!
[FLM]  All files verified successfully.