feat(observers): compare aligned dashboard activity and retained overlap

[n30nex]

Adds Compare with… to the observer dashboard, keeping the selected observer as A. Both activity queries share one explicit UTC anchor and bucket window; a common-axis chart and six-metric table make comparisons readable. The existing raw comparison is labelled retained flood-packet overlap, with its deduplication and expiry limits explained separately from longer-lived summaries.

Comparison selection and time anchor survive shared links/reload. Invalid, self, missing and mismatched-window selections never run a misleading overlap query; changing observers clears the previous totals. English/French text is included. The shared observer reader also accepts the server's existing base64-encoded status JSON, restoring reported noise-floor values without changing the API; malformed metadata stays unknown. Observer freshness polls once per minute through the shared query cache.

Closes #78. Depends on #80 at 04ca3da919d9b0430f364a1a93bf36cf56e9770a (which follows #79/#75), and server #169 for explicit activity windows. Focused change against #80.

Original feature validation on 42ae09b: Windows build/lint and 48 focused observer/API regressions pass; native Pi build/lint/all 895 tests pass. Exact-head build CI passes (upstream CodeQL skipped). Live 24h/7d/30d API totals, bucket sums and payload mix reconcile. Desktop/390-pixel phone, English/French, keyboard, search/sort/scroll/Back, reload and copied/legacy links were checked. The public preview ran that feature candidate, preserving #75/#79/#80 and server #166/#167/#169, with verified assets/source/changelog and rollback. Related #12 remains open for other screens; physical Safari and production-capacity checks remain separate gates.

AI-assisted implementation under n30nex's approved plan and standing authorization; no specific human review of this new diff is claimed. Maintainers control merging and production release.

Current integration (28 September): the queue is refreshed onto accepted web dev 17f48fb9. Parent 04ca3da919d9b0430f364a1a93bf36cf56e9770a, published head 2f76286ec89c81db0345937b8c1712f363bd2e43. Focused diff. Build/lint/tests pass for this refreshed tree and actual-head build CI passes (repository CodeQL skipped). The full web composition 6b688495 is now deployed on Pi server 2ed2e031, retaining every candidate. Its native build/lint and all 935 tests pass; all 18 public assets and both source archives match. Web assets are byte-identical to the previously geometry-tested build; fresh English desktop/French phone and Public-message packet inspection pass. Current evidence and recovery. Earlier feature-specific validation above is historical. No feature behavior changed in this base refresh.

[n30nex]

@MrAlders0n, this observer-release slice is ready for your/Claude's review at 42ae09b7. Web order is #75 → #79 → #80 → #81. This exact candidate passes all 895 native web tests. Shared-window totals reconcile with the live API; retained flood overlap remains separately labelled. Legacy encoded status data, invalid/self selections and stale-result transitions have regressions.

The combined Pi preview is live at https://canadaverse.org/beacon-dev/?tab=Observers with server 88c2c10c / web 42ae09b7, preserving #166/#167/#75. Desktop, 390-pixel phone, English/French, keyboard and navigation checks pass; the changelog and exact source are at https://canadaverse.org/beacon-dev/source.html. Build CI passes (web CodeQL remains skipped). No merges or production switch were performed.

Docs #5 carries the observer-first plan and subsequent connected-investigation work. The proposed public per-IATA MeshMapper scope catalogue is drafted separately, with manual scopes retained; its endpoint/schema is not yet published. The captured historical 1,810 counter difference remains unexplained, not a measured packet-loss claim.