Sprint C: author-filtered stats scan, release cadence + repo size, weekly digest

- src/stats/: OwnEmailMatcher (config emails + GH no-reply per account) +
  git-side --author filtering (cheap even on the pterodactyl fork's 4700+
  commits); streak (current/longest, midnight-grace rule), 52-week heatmap,
  day/hour histograms — one scan feeds all of them, data layer only for now
  (visual Stats-tab redesign stays in Sprint D's mockup-first cycle)
- src/stats/cadence.rs: release cadence (median tag gap, local tags, Tier A)
- src/stats/size.rs + cache.rs: git-object vs working-tree size, persisted
  in new ~/.vulp/stats_cache.toml (max 1 sample/day), recorded only during
  [R]/vulp refresh — confirms vulp's own filter-repo cleanup (539MB->834KB)
- src/digest.rs: WeeklyDigest struct + 3 renderers (Discord embed/markdown
  archive/plaintext); vulp digest [send discord|webhook <url>] CLI, webhook
  stored via existing auth token store (~/.vulp/.tokens, 0600), never logged;
  send respects --dry-run
- TUI: [d] on Stats previews the digest in a Modal::Info before anything is
  ever sent
- fix: list_contributors used --all (double-counts after fetching remotes),
  now --branches
- config: stats_author_emails field (TUI-editable)
- verified: streak/delta/top-repo/release math against a hand-built commit
  history, webhook validation+permissions, dry-run, real refresh (51 repos,
  vulp's own entry confirms the filter-repo win), TUI preview via pyte

src/hub/tabs_stats.rs

@@ -26,7 +26,7 @@ pub fn render_stats_global(f: &mut Frame, state: &HubState, area: Rect) {
     let mut left_lines: Vec<Line> = vec![];
 
     left_lines.push(Line::from(Span::styled(
-        format!("  Global Statistics  [7:{}]  [p] change period", state.global_stats_period.label()),
+        format!("  Global Statistics  [7:{}]  [p] change period  [d] weekly digest preview", state.global_stats_period.label()),
         yel,
     )));
     left_lines.push(Line::from(Span::styled("  ─────────────────────────────────────────────────────────", dim)));