localize on-screen keyboard

[humanapp]

ui-controls: localized on-screen keyboards

Makes the on-screen keyboard localizable. Per-language character sets ride the
existing locale-layer channel, so any consumer gets localized text entry for
free. Fully serves all 16 non-CJK languages, verified on hardware (fr, es, ru).

• text-entry.ts — declares the keyboard _loc charset fields (alphabet,
  accents, symbols) via namespace-extension; resolves them at construction
  (per-modal override › _loc › ASCII default). Layout, case mapping, shift
  visibility, and the input filter derive from the resolved set; the filter fix
  also stops initialText from dropping accents. Adds an accents page (a
  fourth keyboard page, present only when a language has accents) and spreads
  digit/symbol keys evenly across rows.
• locales/charsets.json — keyboard data for 16 languages, glyph-validated
  against font8.
• Additive charset + accents tests; a Spanish samples.ts demo; README section.
• Tooling devDependency + loc:strings via the shared bin; local extractor removed.

Parked as draft until ui-core bump, pending microbit-apps/ui-core#2

[Copilot]

Pull request overview

This PR makes the on-screen text-entry keyboard localizable by introducing locale-driven (and per-modal override) character sets for letters, accents, and symbols, plus an optional accents keyboard page when a language provides accent data.

Changes:

• Extend _loc with keyboard charset fields and resolve an effective charset at text-entry/modal construction time.
• Rework keyboard grid layout to support variable letter row counts and add an accents page that participates in page-cycling.
• Add locale charset data (locales/charsets.json) and smoke tests covering charset injection and accents-page behavior; update sample usage and localization docs.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
text-entry.ts	Adds _loc charset fields, charset resolution, dynamic keyboard layout, and an accents page.
test.ts	Adds smoke tests validating injected charsets, per-modal overrides, caseless behavior, and accents page behavior.
scripts/locstrings.mjs	Removes the local string-extractor script (intended to be replaced by a shared tool).
samples.ts	Localizes a sample caption and demonstrates direct _loc assignment for Spanish/French keyboard behavior.
package.json	Switches loc:strings script and adds a @microbit-apps/ui-core devDependency.
locales/README.md	Documents keyboard charset fields and usage.
locales/charsets.json	Adds per-language keyboard charset data for 16 languages.

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