refactor: generalize input field logic for easier reuse

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (e.g. UI changes), please also provide screenshots/recordings:

This came up while I was reviewing/updating #1953 - we probably want to generalize the really hairy input field logic so it can be reused anywhere. This PR further generalizes it so it's not as tied into app state logic. It also fixes a found bug around deleting whole words if they were were more than one byte long.

Issue

If applicable, what issue does this address?

Closes: #

Testing

If relevant, please state how this was tested (including steps):

If this change affects the program, please also indicate which platforms were tested:

• Windows
• macOS (specify version below)
• Linux (specify distro below)
• Other (specify below)

Checklist

Ensure all of these are met:

• If this PR adds or changes a dependency, please justify this in the description
• If this is a code change, areas your change affects have been linted using (cargo fmt)
• If this is a code change, your changes pass cargo clippy --all -- -D warnings
• If this is a code change, new tests were added if relevant
• If this is a code change, your changes pass cargo test
• The change has been tested to work (see above) and doesn't appear to break other things
• Documentation has been updated if needed (README.md, help menu, docs, configs, etc.)
• There are no merge conflicts
• You have reviewed your changes first
• The pull request passes the provided CI pipeline

Other

Anything else that maintainers should know about this PR:

[codecov]

Codecov Report

❌ Patch coverage is 85.93750% with 81 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.35%. Comparing base (3545a31) to head (fdcc179).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/utils/input.rs	93.02%	36 Missing ⚠️
src/app.rs	0.00%	34 Missing ⚠️
src/canvas/widgets/process_table.rs	0.00%	6 Missing ⚠️
src/widgets/process_table.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2035      +/-   ##
==========================================
+ Coverage   54.04%   55.35%   +1.31%     
==========================================
  Files         122      123       +1     
  Lines       17049    17278     +229     
==========================================
+ Hits         9214     9565     +351     
+ Misses       7835     7713     -122     

Flag	Coverage Δ
macos-14	48.43% <85.76%> (+1.58%)	⬆️
ubuntu-24.04	53.89% <85.93%> (+1.40%)	⬆️
windows-2022	48.06% <85.76%> (+1.63%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.