chore(frontend): remove dead code, consolidate patterns, update README

[Copilot]

The frontend had accumulated unused files, duplicate inline patterns, and an outdated README that no longer reflected the actual structure.

Dead code removed

7 files deleted — none were reachable from any active import path:

File	Reason
types/fileUpload.ts	Empty placeholder
types/maplibre/bounds.ts	computeBoundsFromGeoJSON never called
types/maplibre/priorityStyle.ts	priorityFillColorExpression never used
features/farm/CoordinateFileUpload.tsx	Barrel-exported but never imported
features/farm/YieldFileUpload.tsx	Same
services/fileUploadService.ts	Only consumers were the two deleted components
pages/PrescriptionsPage.tsx	Route was permanently commented out

features/farm/index.ts and AppRoutes.tsx cleaned up accordingly.

Duplicate patterns consolidated

Page gradient background — a 3-line ternary repeated across 6 files extracted into src/utils/theme.ts:

// before (repeated inline in LoginPage, SignupPage, HomePage, ProfilePage, LandingPage, FieldTable)
const gradientBg =
  theme.palette.mode === "dark"
    ? `linear-gradient(180deg, #0a0a0a 0%, ${theme.palette.background.default} 100%)`
    : `linear-gradient(180deg, #f0f0f5 0%, ${theme.palette.background.default} 100%)`;

// after
import { getPageGradientBg } from "../utils/theme";
background: getPageGradientBg(theme)

Error normalization — ProfilePage had a private normalizeErrors doing the same job as normalizeSignupErrors in utils/errors.ts with slightly different join logic. The general version is now exported from utils/errors.ts and ProfilePage imports it from there.

README rewritten

Previous README described an outdated structure (referenced App.tsx, ProtectedRoute in components/, React Router v6). Replaced with accurate documentation covering the current stack, full annotated directory tree, route table, auth flow, conventions, and testing instructions.