Extract i18n to JSON and consolidate page templates

[Victor1890]

What

Replaces 108 duplicated per-locale HTML pages with 5 shared templates + 18 JSON locale files. Fixes nav bugs, flattens engine.js, drops dead code.

Why

Each locale had its own copy of every page. One string change meant editing 18 files — and they had drifted, shipping real bugs:

• window.location.hef typo (9 locales — dead Continue button)
• href=\'...\' backslash escape (8 locales — dead Back button, 5 pages each)
• Orphan page_theme.html linked from nowhere (18 copies)
• Orphan welcome.html unreachable (main.js loads hello.html)

What changed

Area	Before	After
Locale pages	108 HTML in app/lg/<locale>/	5 templates + 18 JSON
i18n	Per-locale HTML duplication	app/i18n/<locale>.json + i18n.js
Navigation	Inline onclick per button	STEPS[] + data-nav orchestrator
commit()	7-deep callback pyramid	Flat readFileSync
Validation	3 fns, regex mismatch vs backend	Single validateUser(), matches post_setup
Dead code	3 orphan files + 18 orphan theme pages	Deleted
index.html	317 lines (6 dead tabs)	39 lines (language select)
finish.html	setTimeout(commit, 10000)	onload="commit()"

Files

Added (24)

• app/i18n/*.json — 18 locale string files
• app/templates/{keymap,timezone,user,agreement,finish}.html
• app/js/i18n.js

Modified (2)

• app/index.html — trimmed to language-select only
• app/js/engine.js — nav orchestrator, flattened commit, unified validation

Deleted (111)

• app/lg/<locale>/*.html (108 files)
• app/js/final-movement.js, app/js/focus-fix.js
• app/welcome.html

Behavior preserved

language select → keymap → timezone → user → agreement → finish → commit() → IPC run-post-setup. Test mode (POST_INSTALL_TEST=1) unchanged. IPC arg order preserved (matches post_setup).

How to test

cd post-install
POST_INSTALL_TEST=1 npm test

Walk: pick language → Continue → keymap → Continue → timezone → Continue → fill user form → Continue → agreement → Continue → finish (test mode closes without running post_setup).