feat(web): theme import/export, more themable colors, hideable local/ical calendars

Theme:
- Export/import themes via UI as <date>_<time>.theme (JSON, readable keys,
  link to THEME.md). Import is partial-aware: only present params are written
  (server accepts partial); prompts before importing files with unknown params.
- Dynamic favicon + theme-color tinted to the primary colour on load/save.
- New themable colours: general hover-highlight, day hover/selected/bg,
  today background, plus two unified sidebar action-icon colours
  (inactive/active) covering bell, hide, delete and read-only icons.
- All new colours are per-setting syncable; documented in THEME.md.

UX:
- Styled confirm dialog (#modal-confirm) replaces window.confirm() for
  calendar delete and account disconnect.
- Birthday/local calendars and iCal subscriptions can now be hidden from the
  sidebar via Settings (new sidebar_hidden column + hide toggle).

Backend: additive nullable columns + idempotent migrations for user_settings
colours and local_calendars/ical_subscriptions.sidebar_hidden.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/main.py

@@ -240,6 +240,29 @@ def _migrate():
             except Exception:
                 pass
 
+        # Fine-grained web element theme colours (all optional overrides).
+        for col in (
+            "hover_highlight_color", "icon_inactive_color", "icon_active_color",
+            "day_hover_color", "day_selected_color",
+            "day_bg_color", "today_bg_color",
+        ):
+            try:
+                conn.execute(text(f"ALTER TABLE user_settings ADD COLUMN {col} VARCHAR(7)"))
+                conn.commit()
+                logging.info("Migration: added %s to user_settings", col)
+            except Exception:
+                pass
+
+        # Allow hiding local (incl. birthday) calendars and iCal subscriptions
+        # from the sidebar, matching caldav/google/ha.
+        for tbl in ("local_calendars", "ical_subscriptions"):
+            try:
+                conn.execute(text(f"ALTER TABLE {tbl} ADD COLUMN sidebar_hidden BOOLEAN DEFAULT 0"))
+                conn.commit()
+                logging.info("Migration: added sidebar_hidden to %s", tbl)
+            except Exception:
+                pass
+
         # CalDAV publishing of local calendars (opt-in, secret token URL).
         for col, ddl in (
             ("caldav_published", "ALTER TABLE local_calendars ADD COLUMN caldav_published BOOLEAN DEFAULT 0"),