Add all-day events, jump-to-today, event editing and ICS escaping

[UncleJ4ck]

Targets the current main (the multi-file rewrite). I checked what the rewrite already has and only added what was missing.

Already in main, so not touched: recurrence (including YEARLY, BYDAY, EXDATE), theme persistence via settings.json, and delete-single-occurrence.

New

• All-day events. Added an allDay field to CalendarEvent. The parser detects a date-only DTSTART, the writer emits DTSTART;VALUE=DATE, the add/edit dialog has an All day toggle, and cards show All day instead of a time range.
• Edit events. Non-recurring manual events get an Edit button that reopens the dialog prefilled. Recurring, imported and khal events are left as-is (editing those safely needs more design around the master/exception model).
• Jump to today button in the header (works in both month and week view).

Fix

_saveManualEventsToDisk wrote SUMMARY/LOCATION/DESCRIPTION raw. A multi-line description (the dialog allows 3 lines) wrote a real line break that the reader dropped on reload, losing everything after the first line. , ; \ were also unsafe. Added RFC 5545 text escaping on write and unescaping on read (SUMMARY/LOCATION are now unescaped too, which helps imported calendars).

Tests

test/widget_test.dart was the placeholder template referencing a non-existent package and class, so it did not compile. Replaced it with unit tests for the ICS escape round-trip and date-only detection.

Verification

• flutter analyze: no new errors or warnings.
• flutter test: passes.
• flutter build linux --debug: builds, launched and rendered on a Wayland session (today button visible).
• The ICS logic is unit-tested. The dialog interactions (edit, all-day toggle) are build and render verified, not click-tested.

Notes

• All-day events use inclusive DTEND dates for internal round-trip simplicity.
• Pre-existing analyzer warnings in the rewrite (unused imports/fields, _sameEvent) are left untouched to keep this focused.