Remove T9 search text-entry option; default to multitap keypad entry

[brickbots]

Summary

Name Search previously offered an optional T9 search mode (the t9_search setting): press each number key once and object names are matched against their keypad-digit sequence. The default — and now the only — behavior is multitap keypad entry: press a key repeatedly to cycle through its letters (7→a/b/c…), building a text string that is matched with a plain case-insensitive substring search.

This PR removes the T9 option entirely and makes multitap the sole text-entry mode.

Changes

• Config: drop the t9_search key from default_config.json.
• Menu: remove the Settings → User Pref → T9 Search item from menu_structure.py.
• UI (textentry.py): remove the t9_search_enabled property and its branches; _perform_search always calls search_by_text, and key_number always uses multitap.
• Catalog (catalogs.py): remove the now-unused search_by_t9 machinery — keypad digit maps (KEYPAD_DIGIT_TO_CHARS, LETTER_TO_DIGIT_MAP), the _t9_cache and its helpers, the cache-invalidation calls, and the now-unused import re. search_by_text is unchanged.
• Tests: delete test_t9_search.py; update test_web_remote_settings.py for the User Pref index shift (Language moves from index 6 → 5, so navigation uses one fewer D).
• Docs: drop T9 references from user_guide.rst and the docs/ax/{catalog,catalog/CONTEXT,ui} references; light wording fixes in the docs skill knowledge base.

Notes

• RELEASE.md (shipped v2.5.1 changelog, already tagged) is intentionally left unchanged — it's a historical record.
• The translation catalogs (locale/*/messages.po) still contain an orphaned T9 Search entry. It's harmless (no longer referenced) and will be removed at the next nox -s babel sync. A full babel run here would have churned ~2,600 lines of unrelated source-line references (the catalogs are refreshed periodically, not per-PR), so it was deliberately left out.

Testing

• pytest -m "smoke or unit" → 281 passed.
• Catalog/text-entry tests → 18 passed, 9 skipped.
• ruff check clean on changed files; mypy reports no errors in the changed modules.
• Web settings test file collects cleanly (Selenium-gated tests skip locally).

🤖 Generated with Claude Code