Show reference-data update time, location filter UI, and tile fixes

- Status gains reference_updated_at (sourced from reference.db's file
  mtime, so it needs no separate persisted state) and the sidebar's
  "Your library" panel shows it as a relative "Updated Xh ago" stamp
  that self-refreshes every minute.
- Add a Location field to the Library tab's filters panel, wired to the
  new LibraryFilter.location.
- Bump every font-size in the stylesheet up one step for a slightly
  larger, easier-to-read UI overall.
- Fix the library/deck tile view's +/- quantity overlay permanently
  covering the card art's own printed mana cost (top-right corner) —
  it now only fades in on hover/focus, like the other row actions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src-tauri/src/commands.rs

@@ -21,14 +21,23 @@ pub struct Status {
     pub reference_path: String,
     pub reference_ready: bool,
     pub card_count: usize,
+    /// Unix timestamp (seconds) of reference.db's last modification — i.e.
+    /// when "Update card data" last completed. `None` if unavailable.
+    pub reference_updated_at: Option<i64>,
 }
 
 fn status_of(s: &AppState) -> Status {
+    let reference_updated_at = std::fs::metadata(&s.reference_path)
+        .and_then(|m| m.modified())
+        .ok()
+        .and_then(|t| t.duration_since(std::time::UNIX_EPOCH).ok())
+        .map(|d| d.as_secs() as i64);
     Status {
         library_path: s.library_path.display().to_string(),
         reference_path: s.reference_path.display().to_string(),
         reference_ready: s.catalog.is_some(),
         card_count: s.index.as_ref().map(|i| i.len()).unwrap_or(0),
+        reference_updated_at,
     }
 }