fix(settings): persist each setting on its own key and sync across windows

[PathGao]

Summary

Every window runs its own SettingsStore instance, and one $effect read about thirty fields and rewrote every localStorage key whenever any of them changed. A second window's store still held the snapshot it was constructed with, so changing one toggle there rewrote all the keys from that stale snapshot.

window A   set font size 20, language 日本語     → keys written
window B   flip "Show Tabs"
           its store still holds the values it
           was constructed with
           the one effect rewrites all ~30 keys  → A's changes overwritten
restart    font size 14, language English

Nothing anywhere listened for storage, so no window ever learned of another's edits.

Persistence is now a table of one entry per key, each installed as its own effect, so an effect only re-runs for the field it owns. A storage listener adopts remote changes through the same entries.

Why compare-and-set rather than a flag

The echo a storage listener invites — adopt a remote value, the local effect fires, write it straight back — is stopped by reading before writing. A flag raised inside the handler is already gone by the time Svelte's asynchronous flush runs the effect, and holding it until the flush would also swallow genuine local edits landing in the same tick. A value that arrived from localStorage already equals what is stored, so the echo dies at its source with no timing assumption at all.

Also fixed

Font sizes reverted on restart. The spin buttons went to 48 while the load path clamped to 24 (editor, code) and 28 (preview): pick 30, restart, get 24. Bounds are single constants now, shared by the UI and the load path, widened to the 48 the UI has been offering — profiles already holding 30 or 40 survive instead of shrinking on upgrade.

Typed values were unbounded, and clearing the field wrote null. HTML min/max only constrain the spinners; an empty field produced font-size: nullpx and stored the string "null". The inputs are one-way now: while typing only in-range values commit — so the "1" of "18" is not clamped to 10 — and blur/Enter/change clamps and writes the corrected value back to the DOM, which is necessary because a clamp landing on the current state produces no re-render to correct the text.

Norwegian and traditional Chinese were never detected. nb and nn are the real BCP-47 primary subtags; startsWith('no') missed both. Matching the exact primary subtag also removes the nl collision by construction. Chinese now reads the script subtag, so zh-Hant, zh-Hant-TW and zh-Hant-HK resolve to traditional.

The dialog lost focus placement. An effect both read and wrote appVersion, so resolving the version re-ran it and re-captured previousActiveElement from inside the dialog. The same shape appeared twice more in that effect (loadFonts() reads the loaded flag it later sets; previousActiveElement was itself reactive). None are rendered, so all three are plain variables now.

Validation

• npm run check — 0 errors, 0 warnings
• npm test — 225/225, including 27 new tests in scripts/settingsPersistence.test.ts

The headline test is "no store field is read by two persisted entries": it runs every entry's read() against a Proxy-recorded store and asserts each reads exactly one property, and that field→entry is a bijection. Svelte's dependency tracking is "which properties did this effect read", so that is a direct proof that changing field A cannot rewrite key B, not a proxy for it. Backed by an end-to-end two-instance simulation of the scenario above.

Note on scripts/previewWidth.test.ts

Two assertions there grep-matched the literal source of the bug — localStorage.setItem('preview.maxWidth', …) inside the giant effect — which no correct fix can preserve. They are replaced with equivalent-intent assertions against that key's persistence entry. The other assertions in the file are untouched and still pass.