feat(keyboard): hold-Enter accent popup; relocate cursor-mode trigger

Replaces the 8 separate Latin-diacritic alt-alphabet pages (Polish, Czech,
Slovak, German, French, Spanish, Portuguese, Nordic) with one popup: Hold
Enter on a Latin letter that has accented variants (a c d e i l n o r s
t u y z) opens a floating horizontal strip of that letter's accents,
anchored over its own row so the grid stays visible underneath (LEFT/
RIGHT picks, Enter inserts via a new shared insertGlyph() helper, Cancel
dismisses). Holding a letter with no variants is a no-op. Cyrillic/Greek
remain full alt-alphabet pages; NodePrefs::keyboard_alt_alphabet shrinks
from 11 to 3 values accordingly -- an old saved Polish..Nordic value just
clamps to Latin via DataStore.cpp's existing range check, no migration
code needed.

Freeing Hold-Enter on letter cells required moving cursor-mode's own
trigger: UP from the top letter row now enters it instead of wrapping to
the special row. To keep that wrap reachable, cursor mode's UP/DOWN
(Home/End) continue the wrap once already at that boundary -- UP again
lands on the special row, DOWN again back on the letter grid -- reusing
the same proportional column mapping the old direct wrap used.

Diagnostics' font-coverage sample swaps its 8 per-language lines for one
line sampling the new accent table. Docs (message_screen, settings_screen,
solo_ui_framework) and release-notes updated to match.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/solo_features/message_screen/message_screen.md

@@ -23,7 +23,7 @@ Press **Enter** on a contact or channel to open its history, then press **Enter*
 - **Custom message** — opens the on-screen keyboard
 - **Q1–Q10** — quick reply templates editable in Settings › Messages
 
-While typing, **Hold Enter** enters cursor mode (LEFT/RIGHT move the insertion point, UP/DOWN jump to start/end, Enter/Cancel exit) so you can edit or insert in the middle of what you've typed instead of only at the end — see the on-screen keyboard section of the [UI framework guide](../../design/solo_ui_framework.md) for the full key set (Shift, T9 multi-tap, alternate alphabets).
+While typing, **UP** from the top letter row enters cursor mode (LEFT/RIGHT move the insertion point; UP/DOWN jump to start/end, then continue on to the special row / letter grid if pressed again once already there; Enter/Cancel exit immediately from anywhere) so you can edit or insert in the middle of what you've typed instead of only at the end. **Hold Enter** on a Latin letter with accented variants (e.g. a, e, c, n, o, s, z…) instead opens a one-row popup of that letter's accents — LEFT/RIGHT to pick, Enter to insert, Cancel to dismiss. See the on-screen keyboard section of the [UI framework guide](../../design/solo_ui_framework.md) for the full key set (Shift, T9 multi-tap, Cyrillic/Greek).
 
 The keyboard supports placeholders that insert live data at send time: