feat(home): section dividers + lint-staged pre-commit

[BK5102]

Summary

Two improvements in one PR:

1. Visual separation between homepage sections. Adds a soft horizontal gradient divider above each .section after the first on the homepage (About / Mission / Programs / Sponsors / Team), so the page reads as distinct chunks.
2. Faster lint pre-commit hook. Replaces the whole-repo pnpm run lint in .husky/pre-commit with lint-staged, which runs ESLint + Prettier on only the files staged for commit.

Why

• The homepage sections previously flowed into each other with only vertical padding, making the transitions feel indistinct. A subtle gradient line gives them clear separation without a heavy hard rule that would clash with the dark theme.
• Running eslint . over the whole repo on every commit was both slow and made every unrelated commit fight pre-existing lint noise elsewhere in the codebase. lint-staged scopes lint+format to only the files in the commit and auto-fixes whitespace/format problems before the commit lands.

Changes

Section dividers (src/app.css)

• .section is now position: relative so its ::before pseudo can anchor to it.
• .section + .section::before renders a 3px-tall, up-to-80rem-wide gradient that fades from transparent to rgba(231,231,229,0.35) at the center and back to transparent.
• Width clamps at 92% so it stays inset from the viewport edges on narrow screens.
• The first .section (About, sitting directly under the Hero) deliberately gets no divider since the Hero already provides visual separation.

Lint-staged pre-commit (package.json, .husky/pre-commit)

• Adds lint-staged to devDependencies and a lint:staged script.
• Adds a lint-staged config block:
  • *.{ts,tsx,js,jsx} → eslint --max-warnings=0 --fix then prettier --write
  • *.{css,md,json,yml,yaml,html} → prettier --write
• .husky/pre-commit now runs pnpm run lint:staged instead of full-repo lint + format:check. Vitest still runs to catch behavioral regressions.
• .husky/pre-push (typecheck + tests) is unchanged.

Reviewer notes

• Run pnpm install once before merging so pnpm-lock.yaml picks up the new lint-staged dev dep. (Lockfile wasn't updated in this PR because the local worktree didn't have a full node_modules tree.)
• Hook still aborts on real lint errors in staged files (--max-warnings=0).

Test plan

Section dividers

• Visit / and confirm a soft horizontal divider sits between About / Mission, Mission / Programs, Programs / Sponsors, and Sponsors / Team
• About has no divider above it (Hero sits above it)
• On mobile the divider stays inside the viewport (clamped to 92%)

Lint-staged

• pnpm install resolves cleanly
• git commit on a small TS/TSX edit runs eslint+prettier on just that file and passes
• git commit with an intentional lint error in a staged file fails
• Editing a file you did NOT stage no longer affects the commit
• git push still triggers pnpm run typecheck and pnpm run test