feat: add TimelineReminder support to TimelinePin

[kchinnasamy]

Summary

Adds an optional reminders: List to TimelinePin (plus a TimelineReminder data class) and the serialization to populate it. Defaults to emptyList(), so existing callers are unaffected.

Motivation

A TimelinePin inserted via insertTimelinePin currently lands in the timeline but is silent — there's no way to attach an alert, so nothing buzzes the watch at the pin's time. Host apps (Core Devices / Cobble) already insert BlobDatabase.Reminder entries for their own calendar reminders; this exposes the same capability to PebbleKit2 consumers so a third-party app's pin can fire a reminder buzz.

Implementation

• common-api/.../TimelinePin.kt — val reminders: List = emptyList() on TimelinePin; new TimelineReminder(time: Instant, layout: TimelineLayout). Use GENERIC_REMINDER layout + system://images/NOTIFICATION_REMINDER icon.
• common/.../TimelinePinSerialization.kt — toPinJson() serializes the reminders array so the host inserts the matching BlobDatabase.Reminder entries.

Fully backward-compatible (empty-list default).

[matejdro]

Nice. This is already read in the Core app, right?

[kchinnasamy]

Nice. This is already read in the Core app, right?

If you mean the vibe-pattern override from #111 — that's merged in microPebble, but the Core-app side of the same change is coredevices/mobileapp#193, still in review, so it hasn't landed in Core yet.

This PR is a different layer, though. #111/#193 control how a reminder buzzes — a global vibration-pattern override applied to the calendar reminders libpebble3 already syncs from the phone. This one is about whether a reminder exists at all for a third-party pin: it adds reminders to TimelinePin so any PebbleKit2 app can attach its own BlobDatabase. Reminder to a pin it inserts and actually buzz, independent of calendar sync. So they're complementary — #14 creates the reminder, #111/#193 style its vibration.