feat: add Spurti Recognition & Growth Hub - achievement gallery, cert…

[kwalgovind1011]

Overview

This PR introduces the Spurti Recognition & Growth Hub — a unified
system that combines recognition, forward-looking growth tracking, and
tangible rewards into the student dashboard.

The design is inspired by how real companies run employee recognition and
rewards programs (e.g. Bonusly, Achievers, Motivosity) — platforms built
around a simple loop: Earn → Track → Showcase → Redeem. Instead of
shipping three unrelated features, this PR ties them into one cohesive
system, each layer feeding into the next.

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🏅 Recognition Layer

Achievement Gallery

• Adds a new "Achievements" tab on the student dashboard showing every
  possible badge (Top 50, Top 10, Consistent Attendee, Poll Champion, Above
  Average, Legend Badge) — not just the ones already earned.
• Locked badges are shown greyed out with a 🔒 icon and their unlock
  criteria, so students know exactly what to work toward next, not just
  what they've already achieved.
• Built on top of the existing buildBadges() logic — added a new
  buildAchievementGallery() function rather than modifying the original,
  so nothing that currently depends on buildBadges() is affected.

Milestone Certificate

• When a student unlocks the Legend Badge (1500+ SP) or reaches Top 10, an
  auto-generated certificate appears — rendered entirely client-side via
  the HTML5 Canvas API (no image assets or server-side rendering needed).
• Includes a "Download Certificate" button (exports as PNG) and a "Share
  on LinkedIn" button (opens a pre-filled LinkedIn share dialog).
• Certificate only renders when the student is actually eligible — this is
  computed from data the profile payload already returns
  (legendBadgeUnlocked, rank), so no new backend logic was needed for
  this part.

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📈 Growth Layer

Weekly Goal Tracker

• Students can set a personal SP target for the current week and see a
  live progress bar (SP earned this week vs. target).
• The "current week" is calculated Monday-based on the backend
  (weekStart() helper), so progress resets automatically without needing
  a cron job or scheduled task.
• Added two new fields to the Student model: weeklyGoalSp and
  weeklyGoalSetAt. A new POST /api/goal endpoint lets a student update
  their target; the response is the full updated profile so the UI stays
  in sync in one round trip.

SP Forecast & Pace Planner

• Projects a student's likely final SP by the end of the internship, based
  on their average SP earned per completed session so far.
• Splits existing Session records into completed vs. remaining (by
  comparing endDateTime to now), and calculates average pace from
  attendance/poll category SPTransaction entries — no new database
  model was needed for this.
• Includes a small interactive calculator: a student enters a target SP
  and immediately sees the pace per remaining session needed to hit it.
  This calculation runs entirely client-side (on data already in the
  profile response) to avoid an unnecessary extra API call.
• Handles edge cases gracefully: zero completed sessions (pace shows 0, no
  crash), and zero remaining sessions (shows "not reachable this cohort"
  instead of dividing by zero).

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🎁 Rewards Layer

SP Redemption Marketplace

• Adds a "Redeem SP" tab where students can trade earned SP for real
  rewards: Resume Review (150 SP), Spurti Swag Kit (100 SP), Course
  Discount Code (200 SP), Priority Mentor Session (250 SP).
• This includes the full backend that was previously missing from an
  earlier UI-only version of this idea — reward costs live in a
  server-side constant (REWARD_CATALOG) so the client can never submit
  its own price, balance is checked server-side before any deduction, and
  the whole redemption is atomic per-request (balance check → deduct →
  log transaction → record redemption).
• New RewardRedemption model tracks what was redeemed, when, and its
  fulfilment status — kept separate from SPTransaction (which just
  records the point movement) so admins get a clean, readable view of
  what students are actually redeeming.
• Every redemption also creates a normal SPTransaction entry (category:
  manual, negative delta), so it shows up correctly in the student's
  existing SP Bank statement — no special-casing needed there.
• New endpoints: GET /api/rewards/catalog (public reward list),
  POST /api/redeem (redeem a reward), GET /api/redeem/history
  (a student's own redemption history).

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Testing

• Manually tested all three layers locally against a seeded test student
  with sample sessions and transactions (past + future sessions, partial
  attendance/poll data).
• Verified zero-data edge cases don't crash: no sessions, no transactions,
  weekly goal not yet set, insufficient SP for any reward.
• Confirmed redemption correctly deducts SP, logs a transaction, and
  reflects the updated balance without a page reload.

Files changed

• server/models/Student.js — added weeklyGoalSp, weeklyGoalSetAt
• server/models/RewardRedemption.js — new model
• server/server.js — forecast calculation in studentPayload; new routes:
  /api/goal, /api/rewards/catalog, /api/redeem, /api/redeem/history
• client/src/main.jsx — new components: AchievementGallery,
  MilestoneCertificate, WeeklyGoalWidget, ForecastWidget,
  RedemptionMarketplace
• client/src/styles.css — styling for all five new components