feat(auth): Add field-level error handling and improved error parsing

[SudiptaPaul-31]

Summary

Correctly parses and displays all backend authentication error responses in the UI. Errors are mapped to user-friendly messages, shown in context (inline field errors or general alert), and automatically cleared when the user starts typing.

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closes #61

Changes

src/features/auth/authThunks.js

• Added parseAuthError helper that maps backend error codes to structured { message, field? } objects
• Mapped codes: EMAIL_NOT_VERIFIED, INVALID_CREDENTIALS, EMAIL_TAKEN, USER_NOT_FOUND, INVALID_TOKEN, TOKEN_EXPIRED
• registerUser and loginUser now call rejectWithValue(parseAuthError(err)) instead of passing the raw error

src/features/auth/authSlice.js

• Added fieldErrors: {} to initial state for field-specific backend errors
• Updated setAuthError to accept both plain strings and { message, field? } objects
• Added clearAuthError action to reset both error and fieldErrors
• registerUser.rejected and loginUser.rejected now populate fieldErrors when the error targets a specific field, otherwise populate general error

src/features/auth/authSelectors.js

• Added selectAuthFieldErrors selector

src/pages/Login.jsx

• Replaced local error / loading state with useSelector(selectAuthError) / useSelector(selectAuthLoading)
• Error alert box now reads from Redux state and has role="alert" for accessibility
• Both inputs dispatch clearAuthError on change
• Dispatches clearAuthError on unmount to prevent stale errors bleeding into other pages
• Fixed broken success check — now uses loginUser.fulfilled.match(result) instead of checking result.payload (which was truthy even on error)

src/pages/Register.jsx

• Reads selectAuthError and selectAuthFieldErrors from Redux
• fieldErrors.email (e.g. EMAIL_TAKEN) renders inline below the email field, falling back to yup validation error
• General backend errors render in a red alert box above the submit button
• handleFieldChange wrapper clears Redux errors on any field interaction
• Dispatches clearAuthError on unmount
• handleCreate uses registerUser.fulfilled.match(result) pattern, removed stale try/catch

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Error Display Behavior

Error code	Where it shows
INVALID_CREDENTIALS	Red alert above submit button
EMAIL_NOT_VERIFIED	Red alert above submit button
EMAIL_TAKEN	Inline below the email field
USER_NOT_FOUND	Inline below the email field
INVALID_TOKEN / TOKEN_EXPIRED	Red alert above submit button

• All errors clear as soon as the user types in any field
• Errors are cleared on page unmount so they don't persist across navigation

[ayshadogo]

Thank you for your effort boss