Add HTML Entity Encoder tool

[GhDj]

Summary

• Add new HTML Entity Encoder/Decoder tool
• Support encoding to named entities (&), numeric entities (&), or hex entities (&)
• Option to encode all characters or only special characters
• Common entities reference panel with clickable items to insert
• Entity reference table organized by categories (special, punctuation, math, currency, arrows)
• Click to copy any entity format from the reference table

Test plan

• All 275 tests pass
• Verify encode/decode functionality works correctly
• Verify encoding mode options (named, numeric, hex) produce correct output
• Verify "encode all characters" option encodes every character
• Verify clicking common entities inserts them into input
• Verify clicking entity formats in the reference table copies them
• Verify dark mode styling

[GhDj]

Merged via cherry-pick into develop