Major dependency upgrades (deferred — break CI)

[AayushCharde]

Dependabot opened these major bumps; each fails CI and needs code/config work, so they were closed for now. Do them deliberately (likely grouped):

• eslint 9 → 10 (chore(deps-dev): bump eslint from 9.17.0 to 10.5.0 in /frontend #50) + eslint-plugin-svelte 2 → 3 (chore(deps-dev): bump eslint-plugin-svelte from 2.46.1 to 3.19.0 in /frontend #40) — flat-config / rule changes; do together
• vite 6 → 8 (chore(deps-dev): bump vite from 6.0.6 to 8.0.16 in /frontend #49) — needs a compatible @sveltejs/vite-plugin-svelte; verify build
• ruff 0.8 → 0.15 (chore(deps-dev): bump ruff from 0.8.4 to 0.15.17 in /backend #46) — new lint rules will fire; fix findings + bump in pyproject target

Dependabot will reopen these unless we close-as-not-planned and/or add ignore/groups in .github/dependabot.yml.