bug: check subtraction when calculating network widget y-axis

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (such as any change that modifies the UI), please provide screenshots of the changes:

This PR adds an explicit check for the case where the Instant is somehow too small and falls back to manually checking for the oldest visible timestamp.

Issue

If applicable, what issue does this address?

Closes: #1825

Testing

If relevant, please state how this was tested. All changes must be tested to work:

If this is a code change, please also indicate which platforms were tested:

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[ClementTsang]

Tested on Windows and it no longer crashes.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.51%. Comparing base (93d49ea) to head (a6302c1).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/canvas/widgets/network_graph.rs	0.00%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1827      +/-   ##
==========================================
- Coverage   43.56%   43.51%   -0.06%     
==========================================
  Files         115      115              
  Lines       16297    16310      +13     
==========================================
- Hits         7100     7097       -3     
- Misses       9197     9213      +16     

Flag	Coverage Δ
macos-14	37.30% <0.00%> (-0.04%)	⬇️
ubuntu-latest	45.10% <0.00%> (-0.06%)	⬇️
windows-2022	37.64% <0.00%> (-0.04%)	⬇️

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[ClementTsang]

On Linux, no functionality change from what I've tested (which is what we want).