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/design/solo_ui_framework.md

@@ -176,15 +176,26 @@ the board's sensors actually provide. Expand them with `expandMsg()` at send tim
 Two layouts share every grid: **ABC** (one key per letter) and **T9**
 (phone-keypad multi-tap — repeated Enter within `KB_T9_TIMEOUT_MS` cycles a
 cell's letter group, then its digit). `NodePrefs::keyboard_alt_alphabet` adds a
-non-Latin alphabet's own page to the cycle (Latin → alphabet → Symbols →
-Latin); each alphabet defines both its ABC grid (`KB_*_CHARS`) and T9 group
-table (`KB_T9_GROUPS_*`) so the two layouts always offer the same letters.
+non-Latin script's own page to the cycle (Latin → alphabet → Symbols → Latin);
+Cyrillic/Greek each define both an ABC grid (`KB_CYRILLIC_CHARS`/
+`KB_GREEK_CHARS`) and a T9 group table (`KB_T9_GROUPS_CYRILLIC`/`_GREEK`) so
+the two layouts always offer the same letters. Latin-diacritic letters (Polish,
+Czech, German, etc.) aren't alt-alphabet pages — they're reached by Hold-Enter
+on a plain Latin cell instead (see `KB_ACCENT_VARIANTS` below).
 Shift is one-shot by default (capitalises the next letter, including whichever
 candidate a T9 multi-tap cycle settles on) or Hold-Enter to toggle caps-lock;
-Hold-Clear erases the whole field. Hold-Enter elsewhere in the field enters
-**cursor mode** (LEFT/RIGHT move the insertion point, UP/DOWN jump to
-start/end) so edits/inserts can target any point in the typed text, not just
-the end.
+Hold-Clear erases the whole field. **UP from the top letter row** enters
+**cursor mode** (LEFT/RIGHT move the insertion point; UP/DOWN jump to
+start/end, then — pressed again once already at that boundary — continue on
+to the special row / letter grid, the same destinations the plain grid wrap
+used to reach directly) so edits/inserts can target any point in the typed
+text, not just the end; Enter/Cancel exit immediately from anywhere.
+Hold-Enter on a Latin-page letter cell with accented variants instead opens
+the **accent popup**: one horizontal row of `KB_ACCENT_VARIANTS[group]`
+(a UTF-8 string per base letter, same shape as a T9 group string), LEFT/RIGHT
+to pick, Enter to insert via the shared `insertGlyph()` helper, Cancel to
+dismiss. Holding a letter with no variants, or any T9/alt-alphabet/symbols
+cell, is a no-op.
 
 `FullscreenMsgView::wrapLines()` is a standalone pixel-accurate word-wrapper
 (O(n), variable-width-font aware) reusable by any multi-line layout; it writes