feat(webapp): console dashboard driven by kb.activity

[plind-junior]

adds a console dashboard as the landing view, modeled on agents-view style session analytics but driven by vouch's own audit log: a 12-month activity calendar (ember heat ramp, themed for dark and light), last-30-days bars, an hour-of-week heatmap, top actors and event mix, plus stat tiles from kb.status / kb.stats. / now redirects to /dashboard instead of chat and the tab leads the sidebar.

the data comes from a new kb.activity read method — one pass over the audit log returning per-day buckets with proposal/decision breakdowns, a 7×24 hour-of-week matrix, and actor/event histograms. kb.audit's tail-limited raw events and kb.stats' review aggregates don't cover this, and shipping raw events to the browser wouldn't scale with the append-only log. registered at all four sites (mcp tool, jsonl handler, capabilities methods, vouch activity cli) plus trust read methods; pure read, no new write path.

windowing is viewer-local calendar days so the oldest heatmap cell is never partially counted (days=0 for all-time, negatives rejected). local bucketing prefers an iana tz name — dst-correct across the year — and falls back to a clamped fixed utc offset. scoping goes through the same viewer context as kb.audit so multi-project kbs don't leak actor names or activity timing across project boundaries.

the view degrades cleanly: endpoints that don't advertise kb.activity get an upgrade hint, empty audit logs get an empty state, and a malformed stats payload can't blank the landing page. the calendar fetches 371 days so its oldest drawn column always sits inside the server window, and chart aria-labels carry totals and peaks for non-mouse users.

note: this branch is based on the local transcript-viewer work (14 commits ending at 776bffb) that wasn't yet on origin/test, so those commits ride along here; the dashboard itself is the last two commits. merge-tree against origin/test is clean, including the bulk-clear merge (#434).

verified on the exact shipped commits in a detached worktree: pytest 1185 passed, mypy and ruff clean, vitest 145 passed, tsc clean; rendered live against a real kb in both themes.

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