i18n: skeleton with Settings language picker and Home extracted (en + zh-CN)

[Janlhin]

Implements the plan agreed in #18.

What's in this PR

• lib/i18n.js — a small, zero-dependency module: t is a Svelte store whose value is the translate function, so $t('key') in templates re-renders on language switch with no wiring. Locale is persisted per device (opensave.locale), defaults to the system language, and any key missing from the active catalog falls back to English, so partial translations never show blanks.
• src/locales/en.json (source of truth) + zh-CN.json — flat, dot-keyed, grouped by view (home.*, settings.*). A plural value is an object keyed by Intl.PluralRules categories ({ "one": …, "other": … }); single-form languages like Chinese are plain strings.
• Settings → Appearance → Language dropdown, defaulting to the system language.
• Home extracted as the first view (both locales), per the agreed scope.
• format.js plural() now selects through the chosen locale's plural rules instead of n === 1.
• Tests (i18n.test.js, 12 cases): catalog completeness both ways (no missing keys, no extra keys, no empty values), English fallback, {param} filling, plural category selection including n defaulting to 0, and locale detection (saved choice > system language > English, storage-refusal safe). Two existing suites that assert English wording now pin locale.set('en') so they don't depend on the machine's language.

npm test: 305/305 (was 249 + these new tests; dev gained tests since v2.4.0).

Notes

• Scope kept to the window UI as discussed; background-service strings, CLI, and the tray/notifications are left for follow-up PRs.
• One bug surfaced while testing: a plural template invoked with no params left {n} unfilled — translate now defaults it to 0 before substitution.
• Follow-ups (after this lands): one PR per view for the remaining views and the home/ components, then the Go-side map for tray/notifications.

Closes #18 (the skeleton step; I'll keep referencing it from the per-view PRs).

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[sivadaboi]

New branch: I've made an i18n branch for this work and pointed this PR at it. Please open the next PRs against i18n instead of dev. That way the translation can land screen by screen, and I'll merge i18n into dev when it's in a good state.

Four small changes before I merge:

1. The catalog tests break on the first plural. leafKeys() walks into plural objects, so { "one", "other" } in en.json becomes k.one + k.other while zh-CN's plain string is k, and all three catalog tests fail. Since the catalogs are flat, comparing top-level keys is enough, with a plural counted as one value that must have an other form.
2. One sentence, one key. The welcome hint is split into hintA/hintB/hintC, which translators can't reorder (and it leaves stray spaces around 设备 and 云备份). Please make it one string with {devices} and {cloud} placeholders, plus a small helper to render those parts in bold. Let's use that rule for every view.
3. Only save the language when someone picks one. Right now the detected language is saved on the first launch, so the app never follows the system language again. Please add a "System default" option at the top of the dropdown too.
4. Set <html lang> to the chosen language. It stays en today, and for Chinese that can make Windows use Japanese glyph shapes for some characters.

Once those are in I'll merge, and you're welcome to start on the next views. Thanks again!

[Janlhin]

All four points are addressed in 5ab46ba:

1. Catalog tests now compare top-level keys only, and a plural value must expose an other form (checked for both catalogs).
2. One sentence, one key: the welcome hint is a single home.welcome.hint with {devices} / {cloud} placeholders, filled via a small segments() helper in lib/i18n.js that renders the named parts in bold — no more hintA/B/C, and no stray spaces. I'll follow this rule for every view.
3. Language is 'system' until explicitly picked: only an explicit pick is persisted (choosing System default removes the saved value), the picker has a "System default" entry on top, and resolveLocale() settles 'system' against the OS language.
4. <html lang> in App.svelte now follows the resolved locale.

npm test: 247/247.

Happy to rebase onto anything else — and I'll open the next PRs against i18n from here on. Thanks for the clear review!