(feat) Add toggle for 3rd row message previews

[brmiller]

Summary

Per Issue #311 , add settings ability to disable message previews

Changes

Adds toggle to Settings | Appearance | Layout | Message List to disable message previews.

Testing

Included index.test.js (not sure if you do test files this way) to test local storage; live compile, re-deploy, re-run, manual testing of toggle on and off, displaying in different layout styles.

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[brmiller]

Yes, I got excited and did not follow all the guidelines in the CONTRIBUTING.md. If you approve the idea of the issue, I will fix up my commit to use conventional commit style. I can also rename my branch if necessary.

[maathimself]

Thanks for this, two things currently fail the frontend test job (I ran npm test on the branch):

1. The store test fails. node --test on store/index.test.js throws ERR_IMPORT_ATTRIBUTE_MISSING for locales/en.json: importing the store transitively pulls in en.json, which Vite handles but raw node --test (what CI uses) rejects without a type: json attribute. That's why there's no existing store test. Please drop store/index.test.js, or, if you want coverage, extract the toggle logic into a small pure helper and test that instead of importing the whole store.

2. The new strings need every locale. The five new admin.messageList.* keys are only in en.json, which fails the existing i18n locale files test (it enforces key parity across locales) and leaves non-English users on English labels. Please add them to the other six (fr, es, it, de, ru, zh-CN).

With those two, npm test goes green. Two more for completeness:

3. Gate the expanded-thread previews too. There are three snippet renders in MessageList.jsx; the PR gates ThreadRow and MessageRow but not the expanded-thread sub-rows (the {msg.snippet || ''} near line 4135). With previews off, expanding a thread still shows a preview per sub-message. Please wrap that one in showMessagePreviews as well.

4. Persist the preference server-side. schedulePrefSave({ showMessagePreviews }) is sent, but PATCH /auth/preferences (backend/src/routes/auth.js) whitelists a fixed set of keys and doesn't include showMessagePreviews, so it's dropped: the toggle only lives in localStorage per device and won't sync across devices. Please add it alongside the sibling prefs (hoverQuickActions, showMobileAvatars) in that handler.

Appreciate you picking this up. Once those are in I'll re-review and get it merged.

[brmiller]

1. The store test fails. node --test on store/index.test.js... Please drop store/index.test.js...

Dropped.

2. The new strings need every locale. ... Please add them to the other six (fr, es, it, de, ru, zh-CN).

I am not a speaker of these languages. Rather than using a plain "google translate", I used an AI with some context of what I was doing and asked it to provide translations. Hopefully this is accurate. If not, let me know how I might get better translations.

3. Gate the expanded-thread previews too. There are three snippet renders in MessageList.jsx; the PR gates ThreadRow and MessageRow but not the expanded-thread sub-rows (the {msg.snippet || ''} near line 4135).

Good catch! I did not know this feature did that as I do not use it. :D Should be fixed.

4. Persist the preference server-side. schedulePrefSave({ showMessagePreviews }) is sent, but ... whitelists a fixed set of keys and doesn't include showMessagePreviews.

Added.