[juev]

Closes #41

What

• Readable HTTP errors. The first line names the error (linkding rejected the API token (HTTP 401), linkding API not found (HTTP 404), otherwise linkding returned HTTP <code>). A JSON body follows as text: detail as is, field errors as field: message. Plain text is kept; an empty body shows only the first line. This text is used on the queue card and in the Test Connection result.
• Notification text. Each persistent sync problem has a short instruction, for example "Saved links stay in the queue. Check the API token in Settings." The notification shows it instead of the server response.
• Save reminder. Test Connection checks the values typed in the fields and does not save them, so a passed test followed by Sync now kept failing with the saved token. After a successful test of a server URL or token that differs from the saved one, Settings shows "Save settings to sync with this connection."
• R4, R22, and R28 in the spec and the Test Connection sentence in README are updated. R22 used to require the full response body; it now requires the same details as text.

Checks

• assembleDebug lintDebug testDebugUnitTest connectedDebugAndroidTest on the API 36 emulator: build and lint pass, 74 instrumented tests pass.
• New tests: ApiErrorsTest, QueueUiTest.testedButUnsavedTokenAsksToSaveSettings; SyncNotifierTest.workerNotifiesRejectedTokenAndClearsAfterSuccess now checks the notification text and the queue card error.