Add ESLint configuration and fix linting issues across monorepo packages

[AnnieIj]

Description

Adds a single, shared ESLint configuration for the whole monorepo and fixes all linting issues so pnpm lint runs cleanly.

The repo already runs ESLint 10 (flat config only — .eslintrc files are no longer supported), so the shared config lives in the root eslint.config.js and is inherited automatically by every workspace package. No per-package configs are needed.

Linked Issue

Closes #339

Type of Change

• 🐛 Bug fix
• ✨ New feature / enhancement
• 📝 Documentation update
• 🔧 Chore / refactor
• 🧪 Tests only

Changes Made

• eslint.config.js — the unified flat config every package inherits:
  • eslint:recommended + typescript-eslint recommended
  • Type-aware linting: every package's tsconfig.json is registered (with tsconfig.base.json as fallback), so rules like no-floating-promises / no-misused-promises now run with full type information
  • consistent-type-imports (error), no-floating-promises (error), no-misused-promises (error), no-non-null-assertion (warn)
  • no-explicit-any kept as a warning (per the issue's suggested config) — visible in the editor without blocking CI
  • Node globals (Buffer, URL, fetch, timers, …) added for .js/.cjs/.mjs files
  • Build artifacts ignored (dist, node_modules, .next, docs build output)
  • no-floating-promises disabled for *.test.ts and test/ helpers: node:test's top-level test()/describe() return promises that the runner itself awaits, so flagging them is a false positive
• Root package.json — added lint:fix script (eslint . --fix)
• apps/access-api & packages/contracts — added missing lint scripts so pnpm -r lint covers every package
• packages/contracts — added prepare script (matches sibling packages) so its dist is built on install and @guildpass/contracts resolves on a fresh clone
• apps/discord-bot — added @types/node (tsconfig declares types: ["node"] but it was missing)
• Fixed lint errors across the repo: unused imports/vars, empty interface extends {} → type alias, unawaited load()/mutate() calls in dashboard pages and hooks (now void/.catch()), async handlers extracted out of onClick props, and import type conversions
• .github/workflows/ci.yml — new CI workflow: installs with pnpm, runs pnpm lint, runs pnpm typecheck, fails on violations

Note on the "no hardcoded .js extensions" criterion: the workspace uses module: NodeNext (ESM), which requires .js extensions on relative imports — removing them would break pnpm build in 5 of 7 packages. The .js extensions in this codebase are correct ESM specifiers, not a linting gap, so enforcing import/extensions: never was intentionally not applied. TypeScript already validates module resolution.

Test Evidence

• pnpm lint — passes across all 7 packages (0 errors; remaining warnings are intentional no-explicit-any / no-non-null-assertion warnings)
• pnpm lint:fix — runs clean
• pnpm typecheck — passes in all 7 packages
• pnpm test — all tests pass (dashboard 47/47, access-api 2/2, webhook-utils)

Checklist

• I have read CONTRIBUTING.md
• This PR is linked to an open issue
• pnpm typecheck passes with no errors
• pnpm lint passes (or issues are pre-existing and documented)
• pnpm test passes (all existing tests still pass)
• The dashboard starts and renders correctly (if UI or API code changed)
• The Discord bot still starts and responds to commands (if bot code changed)
• Docs site renders correctly (if docs changed): pnpm dev:docs
• No secrets, tokens, or API keys are included in this PR
• Environment variable changes are reflected in .env.example
• I have updated documentation if new behaviour was introduced

Additional Notes

The lint fixes in the dashboard are behavior-preserving: load()/mutate() calls now use void or .catch(() => {}) (errors were already surfaced via each mutation's onError → alert), and the two inline async onClick handlers were extracted into named handleInvite/handleCreate functions.

[Lakes41]

This PR cannot be merged automatically because it has merge conflicts.

Please update the branch with the latest base branch and resolve the conflicts.

After the conflicts are resolved and checks pass, the automation can review it again.

[AnnieIj]

@Lakes41 all merged and all CI checked. Thanks