fix(themes): drop malformed installed-theme entries instead of crashing

[dormouse-bot]

Surfaced by the nightly code-quality sweep.

getInstalledThemes() guards against corrupted/tampered localStorage with Array.isArray(parsed), but that only rejects a non-array top level. An array whose elements are malformed — [null], or an object missing id — passes the guard and is returned cast as DormouseTheme[]. The .id dereferences downstream then throw an uncaught TypeError, breaking exactly the theme listing and installation the guard comment says it protects:

• getTheme(id) → getAllThemes().find((t) => t.id === id) → null.id
• addInstalledTheme(theme) / removeInstalledTheme(id) → .filter((t) => t.id !== …) → null.id

This is the same "corrupted or externally tampered storage" case the existing not-an-array test covers, just one level down.

Fix

Filter to well-formed elements (object with a string id) instead of only checking the top-level array. Malformed entries are dropped; well-formed themes still load, matching the graceful-recovery intent.

Test

Adds a regression test feeding [null, { label: 'no id' }, <valid theme>]: it asserts only the valid theme survives and that getAllThemes()/addInstalledTheme() no longer throw. The test fails before this change (the first .map((t) => t.id) throws on null.id).

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