feat(gameplay): implement persistent runs, time-weighted scoring, and resume UX

[rudra496]

Closes #9

Summary of Changes

1. Durable Run State Model & Redux Slice:

   • Defined GameplayRun, GameplayCompletionRecord, and QuestionAnswerRecord types with runtime validation guard isGameplayRun.
   • Implemented gameplaySlice.ts to manage run lifecycle (startRun,
     ecordAnswer, �bandonRun, clearRun).
   • Integrated gameplayReducer into store.ts.

2. Time-Weighted Scoring & Reward Accumulator:

   • Implemented calculateQuestionScore in scoring.ts accounting for mode maxScore, total questions, and elapsed response time ( imeUsed).
   • Wired handleAnswerSubmit in Gameplay.tsx to receive imeUsed from QuestionContainer and update run progression and rewards.

3. Session Persistence & Resume UX:

   • Persisted active in-progress runs under STORAGE_KEYS.GAMEPLAY_RUN in localStorage.
   • Added Resume affordance on /gameplay allowing players to continue interrupted runs or start fresh.
   • Cleared and recorded completions on win/loss without auto-resuming finished runs.

4. Testing & Verification:

   • Added unit test suites for gameplaySlice and scoring under Vitest.
   • Verified clean production build (
     pm run build), linting (
     pm run lint), and 100% test pass rate (
     pm test, 15/15 test suites, 63/63 tests passing).

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[rudra496]

Heads up: the failing test check here isn't from this branch's code. Main's package-lock.json has been broken since #24 — npm ci exits with EUSAGE (missing make-dir/html-escaper/semver and coverage-stack entries), so CI dies at Install before tests even run. Same root cause behind #27's red check.

I opened #30 with a minimal lockfile repair (12 restored entries, zero version changes). Once that merges, this PR's check should run clean.