feat: add GitHub-style activity calendar heatmap to Overview

[jschloman]

Summary

• Adds get_daily_activity(df, swarm_df=None, source="all") to analysis_utils.py — computes zero-filled per-calendar-day activity counts across Last.fm scrobbles and/or Swarm check-ins, using the exact date-normalization patterns already established elsewhere in this codebase.
• Adds a GitHub-contribution-graph-style calendar heatmap to the Overview page (_build_calendar_heatmap_figure() + render_activity_calendar()), with an inline source selector ("All activity" / "Music" / "Check-ins") shown only when Swarm data is actually loaded.
• Rejected the issue's suggested plotly-calplot dependency — confirmed unmaintained (no repo activity in the last month, maintainer stepped away) — in favor of a hand-rolled go.Heatmap using only plotly/pandas, both already declared dependencies. Zero new packages added.
• The selector intentionally omits "Fitness"/"Films" options from the issue's original draft — pages/fitness.py and pages/culture.py are still stub pages with no real backing data source, so those options would be dead UI.
• New CALENDAR_HEATMAP_SCALE constant in components/theme.py (the exact 4-stop indigo→cyan scale from the issue), reused rather than hardcoded inline.
• Zero-fill guarantee: every day in range renders as a real 0-valued cell (not a gap/NaN), so the calendar always looks complete even on sparse data.

Test plan

• New unit tests for get_daily_activity(): zero-fill across internal gaps, multi-source summation, source isolation, None/empty input handling, invalid-source ValueError, tz-naive output guarantee (tests/test_analysis_utils.py)
• New tests asserting directly on the constructed Plotly trace (colorscale, zero-fill z-grid values, hovertemplate) rather than just "a chart rendered" — first such precedent in this codebase (tests/test_overview.py)
• New tests for all 3 source-selector branches, the no-radio-when-no-swarm-data case, and graceful st.info degradation on an empty result
• Full root suite: pytest -n auto → 1048 passed
• Full packages/localizer suite: pytest -n auto → 431 passed
• ruff check . / ruff format --check . — clean
• mypy — clean

Closes #27