fix(mining): clarify folder status legend and add up-to-date marker

[malek10xdev]

Resolves the UI follow-ups from #2882 (merged).

Summary

• Legend for the folder status dots moved from a detached gray sentence into the folders tree, with real color swatches matching the in-tree markers.
• New "Already mined" check marker so a fully mined folder is distinguishable from a never-mined one (previously both had no icon).
• Short, consequence-first labels: "New messages", "Full re-scan needed" (replaces jargon-y "Mailbox identity changed"), "Already mined". Technical explanation lives in per-state tooltips.
• Legend only renders once at least one folder carries a mined status (no noise for first-time users).
• Fully responsive: chips wrap cleanly at narrow widths, dots use shrink-0 so they never squash. FR translations updated.

Testing

Verified locally against GreenMail with a live watermark (5,218-message INBOX):

• Seeded 3 messages → amber dot + count badge updated 5,218 → 5,221.
• Mined via "Continue from last position" → fetcher resumed exactly 5254:5256 (3 messages), completion recorded, watermark advanced to 5256.
• Reopened dialog → status flipped to "Already mined" with check marker.
• Simulated uidvalidity change → red dot + "Full re-scan needed"; restored afterwards.
• ESLint clean on both files; vue-tsc shows no errors in touched files.