Add in-app UI to set site cookies (Fix #2182)

[bchkv]

Category

• a bug fix (Fix #...)
• a new Ripper
• a refactoring
• a style change/fix
• a new feature

Description

Adds Configuration → Site cookies... so users can paste a browser Network tab Cookie header without editing rip.properties manually.

• Cookies are stored in per-domain sidecar files under the config directory (cookies.<domain>), so saving other settings from the UI does not wipe them.
• Still reads legacy cookies.<domain> entries from rip.properties for backward compatibility.
• In-dialog step-by-step instructions (Chrome, Edge, Firefox, Safari).
• 403/401 error messages point users to the new UI.

Related issues:

• Fix Add in-app instructions for how to set cookies for a domain #2182 — in-app cookie instructions + UI (maintainer request).
• Addresses how do you set cookies for a domain? #2179 — documents how to set cookies in the app.
• Addresses Not downloading reddit groups #2180 — Reddit 403 workaround when session cookies are pasted.
• Addresses RedGIFs ripper 401 errors for most links #2167 — Reddit session cookies often reduce RedGIFs 401s when ripping subreddits (not a full RedGIFs API fix).

Testing

Required verification:

• I've verified that there are no regressions in gradlew test (there are no new failures or errors).
• I've verified that this change works as intended.
  • Downloads all relevant content.
  • Downloads content from multiple pages (as necessary or appropriate).
  • Saves content at reasonable file names (e.g. page titles or content IDs) to help easily browse downloaded content.
• I've verified that this change did not break existing functionality (especially in the Ripper I modified).

Optional but recommended:

• I've added a unit test to cover my change.

Notes: SiteCookieStorageTest passes. Full ./gradlew test has live-site rippper failures that also occur on clean main (9 failures on main vs 6 on this branch); none are introduced by the cookie changes.