Standardize user-facing error messages

[DevOlabode]

Current behavior

Server-side messages are already fairly consistent - utils/friendlyError.js and utils/duplicateKeyError.js produce plain-language, punctuated sentences ("Please sign in to continue.", "That username or password doesn't match."). The popup and options page both funnel API errors through a single showStatus(err.message) call (extension/popup/popup.js, ~10 call sites; extension/options/options.js:44).

The actual inconsistency is in the client:

• src/shared/api.js:50 falls back to the literal string 'Request failed' (no period, no context) whenever the server doesn't send an error field - this doesn't match the tone/punctuation of every other message in the app.
• The offline/unreachable-host message (src/shared/api.js:64-67) is a full paragraph with a suggested fix, while every other message is a single short sentence - worth a look, though it may be intentionally more detailed since it's actionable.

Goal

• Give the client a single, consistent fallback message (short sentence, matching the server's style) instead of 'Request failed'.
• Do a quick audit of user-visible strings in popup.js and options.js for tone/punctuation consistency (sentence case, ends with a period, no raw error codes or stack traces reaching the UI).
• Don't change API response shapes or status codes - this is copy/consistency only.

Relevant files

• src/shared/api.js (apiRequest, ApiError)
• extension/popup/popup.js (showStatus and its call sites)
• extension/options/options.js (showStatus)
• utils/friendlyError.js (reference for the existing server-side tone)

Acceptance criteria

• No generic/technical fallback string reaches the UI unexplained
• Client and server error copy read as one consistent voice
• No change to API status codes or response bodies