feat: add --no-view-file flag for interactive viewer (#428)

[ShivamB25]

Summary

• add a new --no-view-file flag and no-view-file config option to disable file content viewing in interactive mode
• block file viewing via both keyboard (v) and mouse double-click, with user feedback and help text updates
• add tests for app wiring and TUI behavior, and update README/configuration/man docs

Testing

• golangci-lint run
• go test ./cmd/gdu/app ./tui
• go test ./...

Closes #428

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.33%. Comparing base (ede21d2) to head (79d1589).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #496      +/-   ##
==========================================
+ Coverage   81.42%   83.33%   +1.91%     
==========================================
  Files          49       49              
  Lines        5126     5156      +30     
==========================================
+ Hits         4174     4297     +123     
+ Misses        816      721      -95     
- Partials      136      138       +2     

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[dundee]

Nice, thanks!