Make compact network speed decimal places configurable

[XSilverTH]

Summary

Add a network_speed_decimal_places setting for sysmon widgets. It controls how many decimal places are shown when compact network speed formatting is enabled

Motivation

Compact network speeds previously were locked to one decimal place. this makes them not look that great when trying to actually keep them compact. i wanted to be able to completely remove that decimal point

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Testing

• Full test suite: 112 passed, 0 failed
• Added formatter tests for 0, 1, and 3 decimal places, rounding, unit boundaries, bytes, and megabytes
• Widget definition tests passed
• clang-tidy passed on changed source files
• Noctalia built successfully
• Translation and formatting checks passed

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I updated user-facing documentation in docs/user/ when this PR changes documented behavior or configuration, or this PR does not require documentation changes.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.

[github-actions]

The description now contains the required template structure.

[SomeFlyingThing]

I completed a follow-up audit and found a desktop show_label regression plus a few validation/visibility gaps. The corrective commit is available as XSilverTH#1, targeting this PR's head branch directly. It also adds regression coverage; the full build and all 113 tests pass.

Once that follow-up is merged, this PR will pick up commit 887b0e99 automatically.