Each repo pair now declares an optional series, grouping every side that
mirrors the same underlying repository (its Forgejo instances and Radicle
node) under one row. The dashboard heatmap becomes one event per cell
(never aggregated behind a single tooltip) inside a collapsible group per
repo, expanding to one sub-row per actual mirrored server with its own
link and its own events - instead of one ambiguous shared link.

Also fixes git commit activity always linking to Forgejo regardless of
which side the content actually landed on, so Radicle now shows up as a
real destination instead of being invisible in the activity feed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KB7C5CeuMbQujqRtb4foki

- Always light (Atlassian-style), no more automatic dark mode
- Each calendar tooltip entry and each recent commit links directly to
the forge issue/PR/commit or the Radicle explorer page it produced
- Fixed the tooltip being clipped by the calendar's horizontal scroll
container (overflow-y: visible instead of the inherited overflow: auto)
- Footer with the source repository link and license
- activity_log now stores a URL per entry; new optional config fields
source_url (footer link) and radicle.explorer_url (Radicle links)

rad issue open --quiet never prints the created issue's id (only a sync
summary), which silently corrupted the stored radicle_id and caused every
issue to be re-mirrored on each pass. Fixed by dropping --quiet and parsing
the id out of the normal output box instead.

Also adds an append-only activity_log table and a small HTTP dashboard
(GitHub-style activity calendar + recent commits list) served alongside
the existing /healthz and /api/status endpoints.

Reworded sentences for clarity and conciseness in the README.