Fix Vietnamese Telex tone/shape-key bugs, correct accent detection for non-Italian locales, seed next-word predictions

[Oxydox]

This PR includes several fixes found and verified while using the Vietnamese Telex layout day-to-day, plus one improvement to the (locale-agnostic) suggestion engine.

Telex fixes (VietnameseTelexProcessor.kt):

Tone marks were landing on the wrong vowel in ao/eo diphthongs (e.g. "nao"+f → "naò" instead of "nào"), and in closed syllables like oan/oat (e.g. "toan"+s → "tóan" instead of "toán").
Non-adjacent letters were incorrectly merging (e.g. typing "d","e","s","t","r" would corrupt into đ duplicating/dropping letters).
Shape keys (a/e/o doubling) could incorrectly reach back through an already-closed syllable coda, corrupting an earlier vowel (e.g. "mam"+"a" → "maam" instead of staying literal).
Added a lightweight "is this even a plausible Vietnamese syllable" check (valid onset/coda validation) with a rollback mechanism, so common English words typed while Telex is active (e.g. "destroyed", "wolf", "straight") no longer get corrupted into nonsense.
Deliberate behavior change: a third press of d on đ now reverts fully to literal dd, matching how the other shape keys already escape (e.g. ô + o → oo). Previously it appended a literal d instead (đd). Confirmed against real device behavior (Gboard/UniKey) before changing. This changes what one existing test (shape keys convert base vowels) expected — updated accordingly.

All changes verified against the full existing test suite (zero regressions) plus a broad set of real Vietnamese words with varied onsets/codas.

Suggestion engine (SuggestionEngine.kt):

The "does this candidate have an accent" check was hardcoded to a small set of Italian-only accented characters, so it silently never fired for the vast majority of Vietnamese diacritics (missed ô, ơ, ư, â, ă, đ, and any combined tone+shape mark). Replaced with the general NFD-based accent-stripping check already used elsewhere in the same file, so it works correctly for any language, not just Italian.

Next-word prediction (UserNGramStore.kt):

Previously started with zero data for every new install — no shipped bigram model at all. Added a small seed set of ~130 common Vietnamese word-pairs, inserted at a low baseline count so real usage naturally overtakes it over time.

[pzauner]

Why closed again?

[Oxydox]

I don't know why the pull request was showing blank for me. I made some updates and just created another one. I am new to this, sorry for the trouble.