feat(sponsor): add one-time sponsor nudge and install prompt

[Rfluid]

Summary

Aura is free and maintained by one person. This adds a gentle, opt-out way for users to support it.

Installer (install.sh)

• After install, prints the GitHub Sponsors and Pix (BRL) links and asks Open the sponsor page now? [y/N] (default No).
• Reads the answer from /dev/tty, so it works under curl | bash. With no terminal attached, it only prints the links.
• Skipped when AURA_NO_SPONSOR_PROMPT or CI is set. Documented in --help.

In-app card

• Seven days after the first run, a card appears under the modal header with Sponsor on GitHub and Pix (BRL) buttons and a × to close it.
• The sponsor buttons only open their link and leave the card up, so someone who paid via Pix can still set up a GitHub sponsorship (or the reverse). Only the × hides it for good.
• first_run and sponsor_nudge_done live in state.json. Users upgrading from an older build start their seven days on the first launch after the upgrade.
• Opt out with aura config set sponsor.nudge false. It is listed in aura config describe.
• The time checks live in aura_core::sponsor, take now as a parameter, and are unit-tested.
• New heart.svg icon. All colors come from theme tokens. Text exists in both the normal and Goblin voices.

Other

• Adds .github/FUNDING.yml (GitHub Sponsors + Livepix), which turns on the repo's Sponsor button.
• SPONSOR.md now lists GitHub Sponsors and Pix.
• Docs updated: README, docs/configuration.md, docs/cli.md, .design/components.md.

Testing

• cargo fmt --check, cargo clippy --workspace --all-targets -D warnings: clean
• cargo test --workspace: 333 passed
• bash -n install.sh; the sponsor prompt function was tested on its own for the skip variables, no terminal, and yes/no/empty answers.
• Not yet checked on screen. To preview the card after rebuilding, run:

  f=~/.local/share/aura/state.json
  jq --arg t "$(date -u -d '8 days ago' +%Y-%m-%dT%H:%M:%SZ)" \
    '.first_run = $t | .sponsor_nudge_done = false' "$f" > "$f.tmp" && mv "$f.tmp" "$f"