fix(themes): guard installed-themes storage against non-array JSON

[dormouse-bot]

Problem

getInstalledThemes() in lib/src/lib/themes/store.ts wraps JSON.parse in a try/catch, so malformed (non-JSON) storage falls back to []. But valid JSON that isn't an array — e.g. localStorage['dormouse:installed-themes'] holding {"id":"oops"} from corruption or external tampering — parses successfully and is returned cast as DormouseTheme[] without any shape check. The non-array value then flows into the spread in getAllThemes() and the .filter(...) in addInstalledTheme() / removeInstalledTheme(), throwing an uncaught TypeError that breaks theme listing and installation entirely.

Found during the nightly rolling survey of lib/src/lib/themes/store.ts.

Solution

Add an Array.isArray guard after the parse: a parsed value that isn't an array now falls back to [], matching the existing fail-soft intent of the surrounding try/catch. No behavior change for well-formed storage.

Testing

This module previously had no test file, though every sibling under themes/ does. Added store.test.ts (@vitest-environment jsdom, reusing the sibling installStorageStub pattern) covering:

• the regression — a valid-but-non-array value now yields [] and doesn't throw through getAllThemes() / addInstalledTheme();
• non-JSON garbage still yields [];
• the install/dedupe-by-id path.

The regression test fails on the pre-fix code (getInstalledThemes() returns {id:'oops'} instead of []) and passes with the guard. tsc --noEmit is clean and all 8 themes/ test files pass locally.

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Automated nightly survey finding.

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