Clean up bugs, threading, and build config

[baz8080]

First of several cleanup PRs based on the analysis of the codebase. Net −15 LOC despite adding the version catalog and Mode/state plumbing.

Summary

Real bugs

• File I/O and SQLite reads/writes moved off the UI thread to Dispatchers.IO (search, favourite reads/writes, clear-all).
• isFavourited() was a SQLite COUNT(*) per row in onBindViewHolder — replaced with a single getFavouriteKeys() upfront and a Set lookup in the adapter.
• MIME typo "plain/text" → "text/plain" on the email-favourites intent.
• onNewIntent now handles INITIAL_WORD for the singleTask activity (previously only the first intent was used).
• Cursor leaks in FavouritesDbHelper fixed with cursor.use { }.
• Per-launch CRC32 over the entire bundled dictionary replaced with a data_version flag in SharedPreferences — re-extract only when bumped.
• linkifyDefinition now trims the cleaned word so onWordClick gets a usable query; linkifySynonyms guards against a missing leading space.

Code smells

• EditText "special:favourites" sentinel replaced with a Mode enum + PreviousState on SearchViewModel; toolbar title now reflects mode.
• Hardcoded English strings (Send mail..., Share word, Shared by Arcus Dictionary, Synonyms:, My Favourites from Arcus Dictionary) moved to strings.xml.
• All app code now lives under com.arcuscomputing.dictionary — the orphan com.arcuscomputing package is gone.
• @Synchronized dropped from FavouritesDbHelper (SQLite handles internal locking); the eager compileStatement cache went with it.
• closeFileHandles() teardown removed from the activity — ArcusApplication owns the dictionary; OS reclaims handles on process death.

Modern paradigms

• <fragment> tag in activity_preferences.xml replaced with FragmentContainerView; the fragment is added programmatically.
• Top-level apply plugin: form replaced with a plugins {} block.
• New gradle/libs.versions.toml version catalog.
• Switched dictionary data dir from getExternalFilesDir to filesDir; removed the now-obsolete "no SD card" dialog and strings.
• TextWatcher interface impl replaced with doAfterTextChanged.

Out of scope (deferred to follow-up PRs)

• Real bug 7 (exit-warning reset in onResume) — left as-is per request.
• Code smell 4 (OFFSET_INDEX / TAGCOUNT_INDEX redundant constants) — left as-is per request.
• Modern paradigm 1 (ViewBinding), 5 (mmap dictionary), 7 (ListAdapter + DiffUtil) — separate PRs.

Test plan

• ./gradlew assembleDebug — passing locally.
• Cold launch: progress dialog briefly shows then dismisses; search works.
• Type "cat" → results appear without ANR; tap a linkified word → searches that word; press back → restores prior query.
• Tap Favourites menu → toolbar title shows "Favourites", list of saved favourites loads. Press back → restores previous search and toolbar.
• Tap a star to (un)favourite — UI flips immediately, persists across re-search.
• In Favourites, sort by date asc/desc and word asc/desc — list reorders.
• Email favourites — intent chooser shows email apps (the text/plain fix).
• Settings activity — preferences screen renders with toolbar + back arrow.
• Reinstall (data wiped) — first launch extracts dictionary into internal storage; second launch skips extraction.

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