Allow user-defined custom regex patterns from the extension UI

[hanneshapke]

Motivation

Custom regex patterns are already supported in the backend config (regex_config.custom_patterns in src/backend/config/pii_detector_config.json), but they can only be configured by editing the JSON file on the server. End users have no way to add domain-specific patterns (employee IDs, internal ticket IDs, project codes, etc.) without filing a request or editing config directly.

We should expose custom regex management through the extension UI so users can add their own patterns at runtime.

Proposal

Add a "Custom patterns" section to the extension options page that lets users define and manage their own regex-based detectors.

UI

• Table of custom patterns with columns: Label, Pattern, Enabled, Actions (edit / delete).
• "Add pattern" form with:
  • Label (e.g., EMPLOYEE_ID) — uppercase, used as the entity type name.
  • Regex pattern (e.g., \\bEMP-\\d{6}\\b).
  • Optional: replacement/mask format.
• Live validation: compile the regex client-side and reject invalid patterns before saving.
• Preview field: paste sample text, see what matches before saving.

Backend

• Endpoint to GET/PUT user-defined patterns, merged with regex_config.custom_patterns from the static config.
• Server-side regex compilation + validation (reject patterns that fail to compile or are catastrophically slow — use Go's regexp which is RE2 and safe from ReDoS, but still cap pattern length).
• User-defined labels appear in the entity-type selection menu alongside model-emitted labels (see Make detected entity types configurable in the extension #458).

Persistence

• Custom patterns are stored per-user (extension storage) and synced to the backend on change.
• Patterns are enabled by default when created.

Acceptance criteria

• Options page has a "Custom patterns" section with add / edit / delete.
• Client-side regex validation with inline error messages.
• Sample-text preview shows matches before saving.
• Backend accepts, validates, persists, and applies user-defined patterns.
• Custom labels appear in the configurable entity-type list (Make detected entity types configurable in the extension #458).
• Invalid or overly long patterns are rejected with a clear error.

Related

• Make detected entity types configurable in the extension #458 — make detected entities configurable (custom patterns should integrate with that menu)
• Existing backend support: src/backend/config/README.md ("Custom Patterns" section)