refactor: Rename overarching_goal to goal

[jhodapp]

Description

Renames all overarching_goal references to goal across the entire frontend codebase — types, API client, SSE event handlers, components, and tests.

This is the first step in a wider refactor and enhancement of goals throughout the platform.

GitHub Issue: Complements backend refactor-platform-rs PR #234

Changes

• Renamed 4 source files via git mv (overarching-goal.ts → goal.ts, overarching-goals.ts → goals.ts, overarching-goal.tsx → goal.tsx, overarching-goal-container.tsx → goal-container.tsx)
• Updated all type definitions: OverarchingGoal → Goal, OverarchingGoalSortField → GoalSortField, SSE event types, EnrichedCoachingSession.overarching_goal → .goal, GoalContext.overarchingGoalId → .goalId
• Rewrote API client: URL /overarching_goals → /goals, OverarchingGoalApi → GoalApi, all SWR hooks renamed
• Updated SSE cache invalidation: event names overarching_goal_* → goal_*, endpoint path /overarching_goals → /goals
• Updated 6 component/consumer files (goal component, goal container, coaching session page, session selector, today session card, join session popover)
• Updated buildGoalContext() and groupActionsByGoal() in assigned-actions utils
• Updated 14 test files (mock paths, mock data fields, variable names)

Testing Strategy

• npx tsc --noEmit — 0 new errors (1 pre-existing in unrelated prototype file)
• npm run lint — 0 errors
• npm test — 58 test files, 596 tests all passing
• grep -ri "overarching" src/ __tests__/ — 0 matches

Concerns

• Must deploy simultaneously with backend PR [Bug]: Overarching goal component overflows beyond the parent container #234. If backend deploys first, frontend will 404 on /overarching_goals API calls and miss renamed SSE events.

[jhodapp]

Closing to re-open as a fresh PR to trigger a clean CI run for the PR Preview pipeline fixes.