fix: show Unknown instead of masked value when entity label missing

[hanneshapke]

Summary

• The PII modal's Type column was showing the masked replacement itself (e.g. `25627`) instead of the entity label (e.g. `Zip`) when the backend response didn't carry a matching label for a given masked value.
• Root cause: `formatEntityType` fell back to stripping `<>` and `_N` from the masked value. That was designed for legacy bracket tokens like `<PERSON_1>`, but for modern realistic-fake replacements (e.g. `25627`) the regex is a no-op and the masked value ends up rendered verbatim as the type.
• Fix: only use the bracket-token fallback when the masked value actually matches `` / `<LABEL_N>`. Otherwise return `Unknown`, so missing labels are at least not misleading.

Before / After

Before (zipcode row):

Type	Found	Replaced with
25627	97204	25627

After (when label is present):

Type	Found	Replaced with
Zip	97204	25627

After (when label is missing for any reason):

Type	Found	Replaced with
Unknown	97204	25627

Test plan

• Load the unpacked extension in Chrome
• Submit a prompt containing a zipcode in ChatGPT and confirm the Type column shows `Zip`, not the masked value
• Submit a prompt with multiple entity types (name, email, zipcode) and confirm each Type is formatted correctly
• Point the extension at a server that does not return `detected_entities` and confirm Type shows `Unknown` (not the masked value)

🤖 Generated with Claude Code