feat: use nouns table for alphabetical autocompletions (#596)

[prince-0408]

Summary

Implements the noun table fallback for alphabetical autocompletions as described in #596. When the autocomplete_lexicon returns no results (e.g. for a new language without a valid Trie structure), the keyboard now falls back to querying the nouns table directly using the language's data contract.

Closes #596

Changes

LanguageDBManager.swift

Method	Type	Description
queryNounAutocompletions(word:)	New	Loads the current language's data contract via ContractManager, extracts all singular key columns from the numbers section, builds a UNION ALL SQL query matching rows where the value starts with the typed prefix (case-insensitive LIKE), and returns up to 3 deduplicated, alphabetically sorted (COLLATE NOCASE ASC) results
queryAutocompletions(word:)	Modified	First queries autocomplete_lexicon as before (existing behaviour preserved) — falls back to queryNounAutocompletions(word:) when the lexicon returns empty results

How It Works Per Language

Each language's YAML contract defines the numbers section differently:

Language	Singular Columns Searched
English, French, Portuguese, Italian	singular
German, Russian	nominativeSingular
Spanish	masculineSingular, feminineSingular
Swedish	nominativeIndefiniteSingular, nominativeDefiniteSingular

The implementation is fully contract-driven — it works automatically for any language that has a valid numbers section, including future languages.

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