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>

THEME.md

@@ -0,0 +1,66 @@
+# Theme parameters (Web)
+
+Calendarr's web client lets you customise the colour theme under
+**Settings → Darstellung → Farben**. Every colour has its own sync toggle (share
+it across your devices or keep it device-local — see
+[backend/SETTINGS_SYNC.md](backend/SETTINGS_SYNC.md)).
+
+You can also **export** the current theme to a `<date>_<time>.theme` file and
+**import** one later. A `.theme` file is plain JSON:
+
+```json
+{
+  "_format": "calendarr-theme",
+  "_version": 1,
+  "_docs": "https://git.scarriffle.com/Scarriffle/Calendarr/src/branch/beta/THEME.md",
+  "exported_at": "2026-07-20T14:33:00.000Z",
+  "settings": {
+    "primary_color": "#4285F4",
+    "hover_highlight_color": "#2A2A38",
+    "...": "..."
+  }
+}
+```
+
+A theme may be **partial** — delete any keys you don't want and only the
+remaining ones are applied. Importing writes **only the parameters present in
+the file** (keys whose sync toggle is on are also pushed to the server; the rest
+update this browser only); omitted parameters are left untouched. If the file
+contains parameters this version doesn't recognise, you're asked whether to
+import the rest anyway.
+
+## Colour parameters
+
+Source of truth for the defaults: `DEFAULT_COLORS` in
+[frontend/js/settings-sync.js](frontend/js/settings-sync.js). Each value is a
+`#RRGGBB` hex string. Where a default is listed as "derived", leaving the value
+untouched reproduces the previous automatic look; setting it overrides that.
+
+| Key | What it colours | Default |
+|---|---|---|
+| `primary_color` | Primary/brand colour — buttons, links, active states, and the browser favicon/tab colour | `#4285F4` |
+| `accent_color` | Accent — danger actions, the "now" line, reminders | `#EA4335` |
+| `today_color` | "Today" accent: the day-number circle and today's labels | `#4285F4` |
+| `text_color` | Base text colour (secondary/tertiary text is derived from it) | `#FFFFFF` |
+| `bg_color` | App background | `#000000` |
+| `surface_color` | Sidebar / top bar / card surfaces (derived from `bg_color` when unset) | `#1A1A1A` |
+| `line_color` | Borders and grid lines | `#3A3A52` |
+| `month_divider_color` | The line marking a month change in the scrolling month view | `#7090C0` |
+| `month_label_color` | The month abbreviation shown at a month change | `#7090C0` |
+| `hover_highlight_color` | General interactive hover — buttons, menu items, list rows | `#2A2A38` (= derived hover) |
+| `icon_inactive_color` | Sidebar action icons (notification bell *off*, hide/eye, delete/trash, "not editable") in their resting / off / not-hovered state | `#9090AA` (= secondary text) |
+| `icon_active_color` | The same sidebar action icons when hovered, pressed, or *on* (e.g. notification bell enabled) | `#E8E8F0` (= primary text) |
+| `day_hover_color` | Hover background over a calendar day (month / week / quarter / agenda / mini-calendar / date picker) | `#2A2A38` (= derived hover) |
+| `day_selected_color` | The selected day — applied as a subtle tint of this colour | `#4285F4` (= primary) |
+| `day_bg_color` | Normal (unselected, non-today) day background. Defaults to the app background so days look transparent | `#000000` |
+| `today_bg_color` | Today's day-cell background — applied as a subtle tint of this colour | `#4285F4` (= today accent) |
+
+Notes:
+- `day_selected_color` and `today_bg_color` are applied as a low-opacity tint of
+  the chosen colour (so content stays readable). The colour you pick is the base
+  hue; the swatch shows the full colour.
+- `day_bg_color` defaults to the app background. If you set a custom
+  `bg_color`, also set `day_bg_color` to match if you want fully transparent days.
+- The other clients (iOS / Android) currently ignore the fine-grained element
+  colours; they are stored and synced by the server but only the web client
+  renders them.